Reintroduced Clean plugin. Refactored Mac and iOS Compile plugin

[ml-preynolds]

The Clean plugin required updating to the 'cocos' namespace. The execution dependency on dist was removed (since that's an optional plugin). The Clean now does the clean with xcodebuild so there's a connection with the Compile plugin class to access the same build environment variables.

Tested compile, run, and clean commands on Mac and iOS

[ml-preynolds]

@natural-law Please consider merging this into v3. Thanks!