Index fixes and performance improvements (fixes #223, #224)

- Fixed creation of duplicate index entries when importing from CurseForge (fixes #224)
- Automatically remove duplicates in index
- Fixed `packwiz serve` with a custom `--pack-root` argument (fixes #223)
- Fixed `packwiz serve` with a custom index.toml location
- Cleaned up internal serving code, added comments and better errors
- Refactored path handling code
- Improved refreshing/exporting performance
- Factored out duplicated exporting logic
- Replaced GetAllMods calls with cleaner LoadAllMods calls and made the former private
- Improved variable names in update command
- Improved handling of aliassed files
- Changed CheckUpdate to take references to metadata
- Removed the ability to use an absolute path to the index file (that probably didn't work anyway)
- Behaviour change: order of entries in exported files may be random

curseforge/curseforge.go

@@ -375,7 +375,7 @@ type cachedStateStore struct {
 	fileInfo *modFileInfo
 }
 
-func (u cfUpdater) CheckUpdate(mods []core.Mod, pack core.Pack) ([]core.UpdateCheck, error) {
+func (u cfUpdater) CheckUpdate(mods []*core.Mod, pack core.Pack) ([]core.UpdateCheck, error) {
 	results := make([]core.UpdateCheck, len(mods))
 	modIDs := make([]uint32, len(mods))
 	modInfos := make([]modInfo, len(mods))