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/import.go

@@ -285,9 +285,8 @@ var importCmd = &cobra.Command{
 		}
 
 		successes = 0
-		packRoot := index.GetPackRoot()
 		for _, v := range filesList {
-			filePath := filepath.Join(packRoot, filepath.FromSlash(v.Name()))
+			filePath := index.ResolveIndexPath(v.Name())
 			filePathAbs, err := filepath.Abs(filePath)
 			if err == nil {
 				found := false