Add support for `sourceMmlcDir`

[grandeljay]

Add source directory for the MMLC:

{
    "sourceDir": "src",
    "sourceDirMmlc": "src-mmlc",
}

Everything in within the sourceDirMmlc directory will be copied to /vendor-no-composer/vendor-name/project-name.

[RobinTheHood]

I like the idea! 👍