Implement a GitUri class

[hach-que]

We have multiple locations in the code base that try to handle parsing shortened Git URLs, but we should centralise the logic so that this parsing can be tested.

Currently the lack of this unified implementation causes uet uefs mount --git-url [email protected]:EpicGames/UnrealEngine not to work correctly. The current workaround is to use uet uefs mount --git-url ssh://[email protected]/EpicGames/UnrealEngine instead.