feat: Extended source support

• feat: Allow full uris
  Useful for external repositories where we do not have any git@ access and always need to clone via a specific protocol

• feat: Support git scheme on repository uri
  For external repositores it might not make sense to depend on the global setting git@ vs. https://. So if the repository has a scheme attached, just use that to override dragon's config

• feat: Add new 'refspec' keywword
  This allows to select a specific commit or branch instead of just using the default from the repository

• feat: Support archive download

• feat: Pass clean environment to subprocesses

• feat: Basic autotools support

• fix: Do not use deprecated urlretrieve function
  Also add hashing so we can skip/verify the download

• feat: Rename doocs-packages to external-dependencies
  Also add tango to it

• feat: Discover dependency on Tango
  This is a bit half-hearted, but works for now

• fixup! feat: Support archive download