feat(librarymanager): allow to use custom providers

[tymonx]

Closes #797

This PR allows to use custom providers to manage FuseSoC libraries.

Additionally:

• Moved the Library class to own fusesoc/library.py module. This is required to avoid circular imports between provider and library manager. The implementation of the Library.update() method was moved to the library manager to break the circular import of the Library class
• Added missing type annotations in providers, library and library manager
• Added missing docstrings in providers, library and library manager
• The library manager is now abstracted from providers (except for the local provider). Replaced strings like checkout that are git specific with update/initialize
• Removed the library_root attribute from the library manager. It was never used anywhere. Dead attribute
• Optimized the LibraryManager.get_library() method for fast lookup by using dictionary instead of list
• Using lazy evaluation in logging. Reason: https://news.ycombinator.com/item?id=44600537

[olofk]

Thanks! This looks like a nice cleanup. The original code was a bit messy so I haven't looked too closely at the logic changes. We should probably have some more tests here to help future changes, but that can be done separately.

A note about library_root. The intention was to provide a way to set the default library location (i.e. fusesoc_libraries) from the command-line or configuration file. It looks like this was never wired up properly though, so I think it's the right call to drop it completely.