fix(venv): Fix all .so files missing when py_binary lives at //:BUILD (#3474)

When `.label.package` was "" (empty string), doing
`"{}/{}".format(ctx.label.package, bin_venv_path)` created an absolute
path,
which caused files to be ignored later, as they no longer looked to be
part
of the correct prefix. An empty package name occurs when the target is
in
`//:BUILD` with venv on.

To fix, use skylib's `paths` instead, which handles it correctly and
won't add the
`/` prefix.

Fixes #3470

Co-authored-by: Shayan Hoshyari <[email protected]>

Author: shayanhoshyari

python/private/venv_runfiles.bzl

@@ -64,7 +64,8 @@ def create_venv_app_files(ctx, deps, venv_dir_map):
         for venv_path, link_to in kind_map.items():
             bin_venv_path = paths.join(base, venv_path)
             if is_file(link_to):
-                symlink_from = "{}/{}".format(ctx.label.package, bin_venv_path)
+                # use paths.join to handle ctx.label.package = ""
+                symlink_from = paths.join(ctx.label.package, bin_venv_path)
                 runfiles_symlinks[symlink_from] = link_to
             else:
                 venv_link = ctx.actions.declare_symlink(bin_venv_path)