Distinguishing different types of strings via type hints

[TomNicholas]

Currently there are a lot of different things which are all type hinted as str, in particular paths (which should probably be pathlib.Path), urls (which we might want to use an external dependency for such as urllib3), and git-related things like tags and hashes.

We want to distinguish all of these via type hints, to make the intent of the code clearer. Otherwise it can get confusing what "source" refers to, for example.