Add support for local source file url scheme

[beenje]

Allow to specify a local file as source:

source:
  - url: file:///build/rich/rich-{{ version }}.tar.gz
    sha256: fb9d6c0a0f643c99eed3875b5377a184132ba9be4d61516a55273d3554d75a39

Fix #176

I never wrote any rust before, so this might require some change :-)

[wolfv]

LGTM! Thanks :)