fix: Add package prefix to uv output

Most log output uses the package name as a prefix (via FromagerLogRecord)
so it is possible to tell which package is being processed when a message
is logged. This was not true when dumping the output of UV and other
external commands to the log, where the entire multi-line output was
logged in a single call.

This fix logs each line of output separately so the package name prefix
is applied to every line, making it much easier to identify which package
is being processed, especially when building multiple packages in parallel
or when reviewing logs.

Changes:
- Log command output line-by-line instead of as a single block
- Skip empty lines to keep logs cleaner
- Apply same pattern to both error and debug output
- Update mock test to provide stdout for proper testing

Fixes: #753

Co-authored-by: Claude <claude@anthropic.com>

Signed-off-by: Ioannis Angelakopoulos <iangelak@redhat.com>

Author: iangelak

src/fromager/external_commands.py

@@ -99,7 +99,17 @@ def run(
         )
         output = completed.stdout.decode("utf-8") if completed.stdout else ""
     if completed.returncode != 0:
-        logger.error("%s failed with %s", cmd, output)
+        # Log the command failure first
+        logger.error(
+            "command failed with exit code %d: %s",
+            completed.returncode,
+            " ".join(shlex.quote(str(s)) for s in cmd),
+        )
+        # Then log each line of output separately so package name prefix is applied
+        for line in output.splitlines():
+            if line.strip():  # Skip empty lines
+                logger.error(line)
+
         err_type = subprocess.CalledProcessError
         if network_isolation:
             # Look for a few common messages that mean there is a network
@@ -113,5 +123,10 @@ def run(
                 if substr in output:
                     err_type = NetworkIsolationError
         raise err_type(completed.returncode, cmd, output)
-    logger.debug("output: %s", output)
+
+    # Log each line of output separately for successful commands too
+    for line in output.splitlines():
+        if line.strip():  # Skip empty lines
+            logger.debug(line)
+
     return output

tests/test_external_commands.py

@@ -29,7 +29,7 @@ def test_external_commands_log_file(tmp_path):
     assert "test\n" == file_contents
 
 
-@mock.patch("subprocess.run", return_value=mock.Mock(returncode=0))
+@mock.patch("subprocess.run", return_value=mock.Mock(returncode=0, stdout=b"test output\n"))
 @mock.patch(
     "fromager.external_commands.network_isolation_cmd",
     return_value=["/bin/unshare", "--net", "--map-current-user"],