Stub Loading is Slow and Repetitive

Currently, loading stubs requires validating the same stubs multiple times. Since the stub JSON file is read from on each validation, this is a very slow and repetitive process. When you have ~5 or more stub packages, executing `micropy` tends to have a very noticeable delay.

A much faster solution would be to only validate when a stub is added (not just loaded from the `stubs` folder) then create and save an md5 hash of the stub info file along with what type of stub it is. We can then "validate" and determine existing stubs by reading from this one file, rather than re-validating them all.

Start by tracing how micropy loads stub packages from the stubs folder and how each stub JSON file is validated. Review the existing add-stub path before changing the loading path. Done means loaded stubs avoid repeated validation while newly added stubs are validated and their type and md5 information are saved for later checks.

Written by the indexing model from the issue text.