Modernize Sample Extensions's AMBuildScript (#2107)

* Modernize Build Scripts

* Fix: Missing dot at extension file name.

Reported by @FortyTwoFortyTwo

* Clean Up: Removed unused variable

* Copying No Longer Needed

Copying the HL2SDK manifest is no longer needed.
Requires latest AMBuild.

* Allow Using Manifests From Project Folder

Check if the manifest already exists in the project folder. If not, then search in the path specified by --hl2sdk-manifest-path.

* Requested Changes by Kenzzer

* Fix Differences Between SDK/No SDK

AMBuildScript

@@ -129,6 +129,8 @@ class SMConfig(object):
       self.mms_root = builder.options.mms_path
     else:
       self.mms_root = ResolveEnvPath('MMSOURCE112', 'mmsource-1.12')
+      if not self.mms_root:
+        self.mms_root = ResolveEnvPath('MMSOURCE', 'metamod-source')
       if not self.mms_root:
         self.mms_root = ResolveEnvPath('MMSOURCE_DEV', 'metamod-source')
       if not self.mms_root: