bazel: rename root BUILD to BUILD.bazel, drop empty WORKSPACE

Match the top-level package convention used by tools/OpenROAD/
(BUILD.bazel at the package root, plain BUILD in sub-packages).

Adding bazel/BUILD in the previous commit turned bazel/ into a
package, so the root BUILD's src reference "bazel/install.sh" became
invalid; point it at the //bazel:install.sh label instead and export
the file from bazel/BUILD.

Drop the empty WORKSPACE — tools/OpenROAD/ already runs WORKSPACE-less
and bazel 8 with no --enable_workspace runs bzlmod-only.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

BUILD BUILD.bazelBUILD renamed to BUILD.bazel

@@ -2,5 +2,5 @@ load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
 
 sh_binary(
     name = "install_for_bazel",
-    srcs = ["bazel/install.sh"],
+    srcs = ["//bazel:install.sh"],
 )

WORKSPACE

@@ -1 +1,3 @@
-exports_files(glob(["*.patch"]))
+exports_files(
+    glob(["*.patch"]) + ["install.sh"],
+)