prefer loading from extension_dir if gem spec is an active stub

[anthraxx]

extension_dir is the modern place to put the native libraries for a gem that is distributed. Try finding and loading the requested library first from this extension dir, in case the library path is covered by an active sub. This makes sure the extension we are trying to load is not just matched based on the gem-name, as there may be multiple versions, but its actually loading a gem spec from a potentially active stub that matches the expected gem version as provided by the gem's dependency constraints.

In case the loader can't find a matching platform library inside an active extension dir, fall back to regular lib path lookup which is glob scanning the ascending path. This makes sure the ffi-compiler loader can also be used for legacy lib paths as well as simple local checkouts and folders.

This is the safest approach that tries to prefer loading from modern extension_dir while still making sure any other use case remains functional.

[anthraxx]

This supersedes #30 with a much simpler approach finding the actual library in question, without making any assumptions about how the library-name relates to the gem-name.
Still fall back to the regular glob based path lookup if this modern fast-path fails.

[davispuh]

Looks good to me! I don't see any reason not to merge this if it works correctly/as expected and no one has any objections.