gn: Use canonical target name for perfetto_proto_library metadata export

The perfetto_proto_library generates some metadata (used by the
gen_bazel and gen_android_bp scripts) that references some target.

In one case, it's generating something like
```
'//protos/perfetto/trace/android/:winscope_common:source_set'
```
which is not the canonical way to represent a target in GN (note the
'/:').

The problem is that we use `get_path_info`, which is for handling paths
and not GN target.

Switching to `get_label_info`, plus some string concatenation, produces
the correct target name.

This allows us to remove a workaround in gn_utils.py, that was
introduced just for this.

This is just an internal cleanup and it has no effect on the generated
build files.

Commit fe05d3a("gen_bazel: handle target names containing '/:'")
bypassed the problem by special casing '/:', but that I think '/:' it's
not a valid gn target name.

Change-Id: I319e8bb7df50cc32ad3da5cd83651fe5a99c025e

gn/proto_library.gni

@@ -372,7 +372,11 @@ template("perfetto_proto_library") {
         metadata = {
           proto_library_sources = invoker.sources
           proto_import_dirs = import_dirs_
-          exports = get_path_info(public_deps_, "abspath")
+          exports = []
+          foreach (i, public_deps_) {
+            # Get the absolute target path
+            exports += [get_label_info(i, "dir") + ":" + get_label_info(i, "name")]
+          }
         }
         forward_variables_from(invoker, vars_to_forward)
       }

tools/gn_utils.py

@@ -176,7 +176,6 @@ def label_to_target_name_with_path(label):
   e.g., //src/perfetto:tests -> src_perfetto_tests
   """
   name = re.sub(r'^//:?', '', label)
-  name = re.sub(r'/:', '/', name)
   name = re.sub(r'[^a-zA-Z0-9_]', '_', name)
   return name