Allow env registry paths to be relative to env file

The default base_url is now derived from the location
of the environment file, so now resource registry paths
can be relative instead of absolute URLs.

This change also includes the following:
* more complete test coverage
* refactor of _process_environment_and_files and
  _get_file_contents to make test coverage easier
* use the full URL instead of just the file name in the
  'files' field to avoid key clashes
* make _get_file_contents less specific to the resource registry
  so it can be used for other inclusion tasks
* stop calling _get_file_contents recursively so that deep URLs
  do not get loaded, instead call _get_file_contents on
  resource_registry and resource_registry.resources.*
Closes-Bug: #1233912

Change-Id: Ic729cbcd1711b89e91daf0d645f4f56bae2accf3
Merge "Allow env registry paths to be relative to env file"