git: Split out LocalRepository implementation

The LocalRepository implementation has grown quite a lot since its inception. Now it feels like it's kind of hard to spot where its relevant parts are, it's mixed with the generic interfaces and thus tends to be a bit awkward to maintain and extend. This MR thus singles out all files which implement LocalRepository into "localrepository_*.go" files and makes it more obvious that the RepositoryRemote and RepositoryConfig types belong to the LocalRepository by renaming them LocalRepositoryRemote and LocalRepositoryConfig.

I didn't yet go all in to single it out into its own package. Instead, I first wanted to make it easier to see what would need to be moved and move it is a second iteration.