Convert the registry to always storing relative paths.

This PR converts the registry to *always* use relative paths. This was a surprising amount of work, but now means:

- KSP installs can be copied and moved without issue.
- Our registry is much smaller.
- We have much stronger checks that where we're working is actually a KSP directory, reducing the risk of errors.
- Our registry has a much more encapsulated format.
- We close #177 .

This PR will automatically detect older registry versions and upgrade them to the new format.

Note that this is a PR onto the release branch, and I swear hand-on-heart (almost) everything here was needed for the bugfix. ;)

Once this is merged I'll merge the `release-v1.0` branch into master (or you can do that if you like).