General improvements and cleanup (mainly for testing)

This MR adds a few improvements, mainly to the tests. These are:

• Separate out integration tests, only run when option set
• Add standardised source tests for git_tag plugin - doing this amounted to copying tests/testutils/repo/git.py and registering git_tag as one
• Migrate format tests missed while migrating plugins
• Add a format test for dpkg_build
• Reorganise where we put the Repo subclasses
• Remove the docker source and its tests
  • It lives in bst-plugins-container now
  • This might be better suited to another MR
• Move package to src/

This does gives us standardised source tests for all sources that can really use them, as the quilt source just applies patches from an already downloaded source. I suppose it could technically close #6 (closed)