Saner handling of detected DLLs.

- We no longer register DLLs which are part of a mod.
- Registry.InstalledVersion now reports provided modules.
- Registry.Installed now returns provided modules as well.
- Documentation and clean-up of the Registry class.

Note that this PR includes some file renames, so that all
registry C# files live in their own directory.

`git diff 0183a2b..1b10ad7` will show changes excluding the rename.

Closes #141.