Add migrate command (#207)

* Add base and loader command

* Support updating to latest

* Support explicit fabric versions

* Support explicit Forge version

* Support quilt

* Add support for updating Minecraft

* Add support for Forge recommended

* Fix Forge version sorting

* Various fixes

* Add documentation to the use

* More suggestions

cmdshared/mcversion.go

@@ -0,0 +1,52 @@
+package cmdshared
+
+import (
+	"encoding/json"
+	"fmt"
+	"github.com/packwiz/packwiz/core"
+	"os"
+	"sort"
+	"time"
+)
+
+type McVersionManifest struct {
+	Latest struct {
+		Release  string `json:"release"`
+		Snapshot string `json:"snapshot"`
+	} `json:"latest"`
+	Versions []struct {
+		ID          string    `json:"id"`
+		Type        string    `json:"type"`
+		URL         string    `json:"url"`
+		Time        time.Time `json:"time"`
+		ReleaseTime time.Time `json:"releaseTime"`
+	} `json:"versions"`
+}
+
+func (m McVersionManifest) CheckValid(version string) {
+	for _, v := range m.Versions {
+		if v.ID == version {
+			return
+		}
+	}
+	fmt.Println("Given version is not a valid Minecraft version!")
+	os.Exit(1)
+}
+
+func GetValidMCVersions() (McVersionManifest, error) {
+	res, err := core.GetWithUA("https://launchermeta.mojang.com/mc/game/version_manifest.json", "application/json")
+	if err != nil {
+		return McVersionManifest{}, err
+	}
+	dec := json.NewDecoder(res.Body)
+	out := McVersionManifest{}
+	err = dec.Decode(&out)
+	if err != nil {
+		return McVersionManifest{}, err
+	}
+	// Sort by newest to oldest
+	sort.Slice(out.Versions, func(i, j int) bool {
+		return out.Versions[i].ReleaseTime.Before(out.Versions[j].ReleaseTime)
+	})
+	return out, nil
+}

cmdshared/utils.go

@@ -0,0 +1,16 @@
+package cmdshared
+
+import "strings"
+
+func GetRawForgeVersion(version string) string {
+	var wantedVersion string
+	// Check if we have a "-" in the version
+	if strings.Contains(version, "-") {
+		// We have a mcVersion-loaderVersion format
+		// Strip the mcVersion
+		wantedVersion = strings.Split(version, "-")[1]
+	} else {
+		wantedVersion = version
+	}
+	return wantedVersion
+}