feat(rules): add build_data_file field to PyExecutableInfo (#2181)

PyExecutableInfo was added in
https://github.com/bazelbuild/rules_python/pull/2166 with the field
`runfiles_without_exe` that intentionally excludes files that are
specific to
that target/executable, such as the build data file (which may contain
the target name,
or other target-specific information).

However, consuming tools (such as ones used within Google) may need to
derive a file from
that build data, override it completely, or be happy with its content as
is. To aid that
case, expose it via PyExecutableInfo.

python/private/common/py_executable.bzl

diff --git a/python/private/common/py_executable.bzl b/python/private/common/py_executable.bzl
index 1437e2e..80418ac 100644
--- a/python/private/common/py_executable.bzl
+++ b/python/private/common/py_executable.bzl
@@ -426,6 +426,8 @@
         * data_runfiles: The data runfiles
         * runfiles_without_exe: The default runfiles, but without the executable
           or files specific to the original program/executable.
+        * build_data_file: A file with build stamp information if stamping is enabled, otherwise
+          None.
     """
     common_runfiles_depsets = [main_py_files]
 
@@ -465,17 +467,20 @@
     data_runfiles = runfiles_with_exe
 
     if is_stamping_enabled(ctx, semantics) and semantics.should_include_build_data(ctx):
-        default_runfiles = runfiles_with_exe.merge(_create_runfiles_with_build_data(
+        build_data_file, build_data_runfiles = _create_runfiles_with_build_data(
             ctx,
             semantics.get_central_uncachable_version_file(ctx),
             semantics.get_extra_write_build_data_env(ctx),
-        ))
+        )
+        default_runfiles = runfiles_with_exe.merge(build_data_runfiles)
     else:
+        build_data_file = None
         default_runfiles = runfiles_with_exe
 
     return struct(
         runfiles_without_exe = common_runfiles,
         default_runfiles = default_runfiles,
+        build_data_file = build_data_file,
         data_runfiles = data_runfiles,
     )
 
@@ -483,15 +488,15 @@
         ctx,
         central_uncachable_version_file,
         extra_write_build_data_env):
-    return ctx.runfiles(
-        symlinks = {
-            BUILD_DATA_SYMLINK_PATH: _write_build_data(
-                ctx,
-                central_uncachable_version_file,
-                extra_write_build_data_env,
-            ),
-        },
+    build_data_file = _write_build_data(
+        ctx,
+        central_uncachable_version_file,
+        extra_write_build_data_env,
     )
+    build_data_runfiles = ctx.runfiles(symlinks = {
+        BUILD_DATA_SYMLINK_PATH: build_data_file,
+    })
+    return build_data_file, build_data_runfiles
 
 def _write_build_data(ctx, central_uncachable_version_file, extra_write_build_data_env):
     # TODO: Remove this logic when a central file is always available
@@ -829,6 +834,7 @@
         PyExecutableInfo(
             main = main_py,
             runfiles_without_exe = runfiles_details.runfiles_without_exe,
+            build_data_file = runfiles_details.build_data_file,
             interpreter_path = runtime_details.executable_interpreter_path,
         ),
     ]

python/private/py_executable_info.bzl

diff --git a/python/private/py_executable_info.bzl b/python/private/py_executable_info.bzl
index 7fa2f18..deb1194 100644
--- a/python/private/py_executable_info.bzl
+++ b/python/private/py_executable_info.bzl
@@ -10,6 +10,11 @@
 :::
 """,
     fields = {
+        "build_data_file": """
+:type: None | File
+
+A symlink to build_data.txt if stamping is enabled, otherwise None.
+""",
         "interpreter_path": """
 :type: None | str