Partial fix for running udc-qpr-dev with host_substitution=all

This accommodates the different file names in udc-qpr-dev.

There is an outstanding issue that run_cvd passes new flags to crosvm
that don't exist in the udc-dev version of crosvm, specifically:

```
log_tee(2421652)  I 03-20 15:54:15 2421652 2421652 log_tee.cpp:167] [2026-03-20T15:54:15.353288776-07:00 INFO  crosvm] crosvm started.
log_tee(2421652)  E 03-20 15:54:15 2421652 2421652 log_tee.cpp:158] [2026-03-20T15:54:15.353444901-07:00 ERROR crosvm] arg parsing failed: Unrecognized argument: --vhost-user
log_tee(2421652)  E 03-20 15:54:15 2421652 2421652 log_tee.cpp:158]
log_tee(2421652)  E 03-20 15:54:15 2421652 2421652 log_tee.cpp:158] [2026-03-20T15:54:15.353464208-07:00 INFO  crosvm] invalid argument
run_cvd(2421601)  I 03-20 15:54:15 2421604 2421604 process_monitor.cc:81] Detected unexpected exit of monitored subprocess /var/tmp/cvd/270178/1774047250279517/artifacts/host_tools/bin/process_restarter
run_cvd(2421601)  I 03-20 15:54:15 2421604 2421604 process_monitor.cc:83] Subprocess /var/tmp/cvd/270178/1774047250279517/artifacts/host_tools/bin/process_restarter (2421653) has exited with exit code 35
log_tee(2421668)  I 03-20 15:54:15 2421668 2421668 log_tee.cpp:167] [2026-03-20T15:54:15.356759303-07:00 INFO  crosvm] crosvm started.
log_tee(2421668)  E 03-20 15:54:15 2421668 2421668 log_tee.cpp:158] [2026-03-20T15:54:15.356936379-07:00 ERROR crosvm] arg parsing failed: Unrecognized argument: --vhost-user-connect-timeout-ms
log_tee(2421668)  E 03-20 15:54:15 2421668 2421668 log_tee.cpp:158]
log_tee(2421668)  E 03-20 15:54:15 2421668 2421668 log_tee.cpp:158] [2026-03-20T15:54:15.356955304-07:00 INFO  crosvm] invalid argument
run_cvd(2421601)  I 03-20 15:54:15 2421604 2421604 process_monitor.cc:81] Detected unexpected exit of monitored subprocess /var/tmp/cvd/270178/1774047250279517/artifacts/host_tools/bin/process_restarter
run_cvd(2421601)  I 03-20 15:54:15 2421604 2421604 process_monitor.cc:83] Subprocess /var/tmp/cvd/270178/1774047250279517/artifacts/host_tools/bin/process_restarter (2421679) has exited with exit code 35
run_cvd(2421601)  E 03-20 15:54:15 2421604 2421604 process_monitor.cc:143] Stopping all monitored processes due to unexpected exit of critical process
```

Repro command:
```
bazel run //cuttlefish/package:cvd -- fetch \
  --default_build=git_udc-qpr-dev/cf_x86_64_phone-userdebug \
  --target_directory=$HOME/dl_udc \
  --host_substitutions=all

bazel run //cuttlefish/package:cvd -- create \
  --host_path=$HOME/dl_udc \
  --product_path=$HOME/dl_udc \
  --host_substitutions=all
```

Bug: b/494662475

Author: Databean

base/cvd/cuttlefish/host/commands/cvd/fetch/substitute.cc

@@ -61,6 +61,8 @@ Result<void> Substitute(const std::string& target,
     return {};
   }
 
+  CF_EXPECT(EnsureDirectoryExists(android::base::Dirname(full_link_name)));
+
   if (FileExists(full_link_name)) {
     CF_EXPECTF(unlink(full_link_name.c_str()) == 0, "{}", StrError(errno));
   }
@@ -80,17 +82,15 @@ Result<void> SubstituteWithFlag(
   if (host_substitutions == std::vector<std::string>{"all"}) {
     auto callback = [&bin_dir_parent,
                      &target_dir](const std::string& path) -> Result<void> {
+      if (IsDirectory(path)) {
+        return {};
+      }
       std::string_view local_path(path);
       CF_EXPECTF(android::base::ConsumePrefix(&local_path, bin_dir_parent),
                  "Unexpected prefix in : '{}'", local_path);
 
       const std::string to_substitute = target_dir + std::string(local_path);
-      if (FileExists(to_substitute) && !IsDirectory(to_substitute)) {
-        CF_EXPECTF(unlink(to_substitute.c_str()) >= 0,
-                   "Failed to unlink '{}': '{}'", to_substitute,
-                   StrError(errno));
-        CF_EXPECT(Symlink(path, to_substitute));
-      }
+      CF_EXPECT(Substitute(path, to_substitute));
       return {};
     };
     CF_EXPECT(WalkDirectory(bin_dir_parent, callback));

base/cvd/cuttlefish/host/libs/config/BUILD.bazel

@@ -367,7 +367,10 @@ cf_cc_library(
     name = "known_paths",
     srcs = ["known_paths.cpp"],
     hdrs = ["known_paths.h"],
-    deps = ["//cuttlefish/host/libs/config:config_utils"],
+    deps = [
+        "//cuttlefish/common/libs/utils:files",
+        "//cuttlefish/host/libs/config:config_utils",
+    ],
 )
 
 cf_cc_library(

base/cvd/cuttlefish/host/libs/config/known_paths.cpp

@@ -18,6 +18,7 @@
 
 #include <string>
 
+#include "cuttlefish/common/libs/utils/files.h"
 #include "cuttlefish/host/libs/config/config_utils.h"
 
 namespace cuttlefish {
@@ -28,7 +29,14 @@ std::string AutomotiveProxyBinary() {
   return HostBinaryPath("automotive_vsock_proxy");
 }
 
-std::string AvbToolBinary() { return HostBinaryPath("avbtool.py"); }
+std::string AvbToolBinary() {
+  const std::string py_ext = HostBinaryPath("avbtool.py");
+  if (FileExists(py_ext)) {
+    return py_ext;
+  } else {
+    return HostBinaryPath("avbtool");
+  }
+}
 
 std::string CasimirBinary() { return HostBinaryPath("casimir"); }
 
@@ -156,15 +164,25 @@ std::string TestKeyRsa2048() {
 }
 
 std::string TestKeyRsa4096() {
-  return DefaultHostArtifactsPath("etc/cvd_avb_testkey_rsa4096.pem");
+  const std::string rsa_4096 = DefaultHostArtifactsPath("etc/cvd_avb_testkey_rsa4096.pem");
+  if (FileExists(rsa_4096)) {
+    return rsa_4096;
+  } else {
+    return DefaultHostArtifactsPath("etc/cvd_avb_testkey.pem");
+  }
 }
 
 std::string TestPubKeyRsa2048() {
   return DefaultHostArtifactsPath("etc/cvd_rsa2048.avbpubkey");
 }
 
 std::string TestPubKeyRsa4096() {
-  return DefaultHostArtifactsPath("etc/cvd_rsa4096.avbpubkey");
+  const std::string rsa_4096 = DefaultHostArtifactsPath("etc/cvd_rsa4096.avbpubkey");
+  if (FileExists(rsa_4096)) {
+    return rsa_4096;
+  } else {
+    return DefaultHostArtifactsPath("etc/cvd.avbpubkey");
+  }
 }
 
 std::string TombstoneReceiverBinary() {