Initial work on 1.13 forge resolver, fixes.

Moved java executable to its own util class.
Fixed artifact resolution links for older versions of forge.

1.13 support is going to be difficult because forge does not
make anything developer friendly.

src/util/PackXZExtractWrapper.ts

@@ -1,12 +1,9 @@
 import { spawn } from 'child_process'
 import { join } from 'path'
+import { JavaUtil } from './javautil'
 
 export class PackXZExtractWrapper {
 
-    public static getJavaExecutable() {
-        return process.env.JAVA_EXECUTABLE as string
-    }
-
     public static getPackXZExtract() {
         return join(process.cwd(), 'libraries', 'java', 'PackXZExtract.jar')
     }
@@ -25,7 +22,7 @@ export class PackXZExtractWrapper {
 
     private static execute(command: string, paths: string[]) {
         return new Promise((resolve, reject) => {
-            const child = spawn(PackXZExtractWrapper.getJavaExecutable(), [
+            const child = spawn(JavaUtil.getJavaExecutable(), [
                 '-jar',
                 PackXZExtractWrapper.getPackXZExtract(),
                 command,

src/util/javautil.ts

@@ -0,0 +1,7 @@
+export class JavaUtil {
+
+    public static getJavaExecutable() {
+        return process.env.JAVA_EXECUTABLE as string
+    }
+
+}

src/util/versionutil.ts

@@ -17,7 +17,7 @@ export class VersionUtil {
                 }
             }
         }
-        return null
+        throw new Error(`${version} is not a valid minecraft version!`)
     }
 
 }