Make repo names case-insensitive

[Jackenmen]

Description of the changes

This PR makes repo names in Downloader case-insensitive. This involves:

• making sure that the name of the repo is lowercase everywhere relevant (at API boundaries)
• adding a migration that updates all repos to use lowercase names, resolving any conflicts that may occur
  • RepoManager's Config is completely replaced by a config with a new identifier and its repos folder is completely replaced by a new (repos.v2) folder - this allows us to create a mapping of old names to new names and then perform a rename in any order without overriding any existing data

Testing of this PR's changes should involve:

• testing the migration, in particular:
  • name conflict handling (i.e. add 2 repos with names name and NAME and try performing a migration)
    • note for testers: while I'd generally only expect this to be possible on Linux/macOS file systems, it might be possible to test this on Windows too by creating an empty directory as case-sensitive and then creating an instance with that dir as data path, see: https://learn.microsoft.com/en-us/windows/wsl/case-sensitivity#modify-case-sensitivity; might require WSL feature to be enabled though
    • It would be good to test that Downloader prefers not changing existing names, i.e. with 3 repos named name, NAME, and name-2, I'd only expect NAME to be renamed to name-3, leaving name-2 as is
  • handling of repo names that were taken from installed cogs are have since been removed
  • exiting the migration mid-run (probably by just adding a line raising an exception in some strategic place in the repo manager migration)
  • checking that the config JSONs don't have extraneous data left after the migration and that data that is there is in the correct format post-migration
• checking that all commands correctly work when passing an uppercase name for a repo, i.e. JackCogs should always manage to find a repo named jackcogs

Fixes #6371, fixes #6631, blocked by #6741

Have the changes in this PR been tested?

No