During implementation of RHELAI-2984, several things were noticed with the package_plugins calls to resolve_source().

• the gradlib code has a unique call to _resolve_specifier() which now can be a call to the utils.py:resolve_specifier()
• both the gradlib and vllm code use DOWNLOAD_RELEASE strings instead of REPO strings. In order to make the code common, the use of DOWNLOAD_RELEASE strings can be dropped.
• some calls to each package's _get_version_info() function pass the requirement into the function even though it isn't used. These can be cleaned up.

As a result of these cleanups, the resolve_source() override function can be simplified in most cases to a single common call of resolve_specifier().