Add plugin for local files with ref

The local plug-in is some times needed for files that are not available remotely.

The local plug-in ensures there cache key of such files by hashing them on the invocation of bst.

For large files were hashing them is costly this has a large performance hit as every invocation of bst that needs to known the elements cache key requires this hashing.

This plug-in only calculates the hash when the files are added to the source cache in the same way as remote sources work.

For a developer work flow this sources state can be cached if it is already in the source cache and for a CI work flow a sources state can be resolved and a artefacts cache key can be calculated and retrieved from a cache without having to hash all the local files.