Implement pack importing/exporting for downloaded Curseforge packs

Abstract out hash implementations
Implement file saving/downloading

curseforge/packinterop/interfaces.go

@@ -0,0 +1,25 @@
+package packinterop
+
+import "io"
+
+type ImportPackFile interface {
+	Name() string
+	Open() (io.ReadCloser, error)
+}
+
+type ImportPackMetadata interface {
+	Name() string
+	Versions() map[string]string
+	// TODO: use AddonFileReference?
+	Mods() []struct {
+		ModID  int
+		FileID int
+	}
+	GetFiles() ([]ImportPackFile, error)
+}
+
+type ImportPackSource interface {
+	GetFile(path string) (ImportPackFile, error)
+	GetFileList() ([]ImportPackFile, error)
+	GetPackFile() ImportPackFile
+}