[build][tools] Remove `protobuf_dart` template and exec_script usage

This CL migrates
`tools/dart-strict-deps:dart_strict_deps_proto` away from the
`protobuf_dart()` template which uses exec_script.
dart_strict_deps_proto is the only target that uses this template.

The template itself will be deleted in a followup CL.

This CL also deletes the `label_to_package_name.py` script which is
invoked by the exec_script instance in protobuf_dart.

Change-Id: I7fc1bdac40425c3c238f1dc91c1e8ce8f35dff7e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/477861
Commit-Queue: Nic Zoghb <nzo@google.com>
Fuchsia-Auto-Submit: Nic Zoghb <nzo@google.com>
Reviewed-by: Shai Barack <shayba@google.com>