fix: work around version parsing by only parsing if site-packages is enabled (bazel-contrib#3031)

There's a bug in the version string parser that doesn't handle local
identifiers correctly.

Thankfully, it's only activated in the experimental code path when site
packages for
libraries is eanbled. Moving the logic within that block works around
it.

Work around for
bazel-contrib#3030

Author: rickeylev

python/private/py_library.bzl

@@ -161,8 +161,7 @@ def py_library_impl(ctx, *, semantics):
     imports = []
     venv_symlinks = []
 
-    package, version_str = _get_package_and_version(ctx)
-    imports, venv_symlinks = _get_imports_and_venv_symlinks(ctx, semantics, package, version_str)
+    imports, venv_symlinks = _get_imports_and_venv_symlinks(ctx, semantics)
 
     cc_info = semantics.get_cc_info_for_library(ctx)
     py_info, deps_transitive_sources, builtins_py_info = create_py_info(
@@ -241,10 +240,11 @@ def _get_package_and_version(ctx):
         version.normalize(version_str),  # will have no dashes either
     )
 
-def _get_imports_and_venv_symlinks(ctx, semantics, package, version_str):
+def _get_imports_and_venv_symlinks(ctx, semantics):
     imports = depset()
     venv_symlinks = []
     if VenvsSitePackages.is_enabled(ctx):
+        package, version_str = _get_package_and_version(ctx)
         venv_symlinks = _get_venv_symlinks(ctx, package, version_str)
     else:
         imports = collect_imports(ctx, semantics)