Install dir fixes

Closes #48 (closed).

Adds InstallDir.DATA_OVERRIDES, which is used to allow a secondary InstallDir (such as for a mesh compatibility patch) to override certain other mods without forcing the default install directory to also override the other mods. To allow us to break cycles, any InstallDirs with DESTPATHs other than the default are treated separately in the graph when sorting. DESTPATH has been renamed to PATCHDIR to make it more clear that it is to be used to specify a directory containing a patch that can be included at a different point in the load order.

Adds InstallDir.RENAME, which is used to handle directories that don't have the right parent structure such as textures that are included in an optional directory without being enclosed in a Textures directory. If RENAME and PATCHDIR are both used, the final path becomes PATCHDIR/RENAME.

This also fixes the issue where multiple install directories with the same destination path were showing up multiple times in the config.

Tested and issues have been fixed. assets/mesh-fix was used for the test and will be updated shortly.