Move source from 'buildstream' to 'src/buildstream'

• Move source from 'buildstream' to 'src/buildstream'

  This was discussed in #1008 (closed).

  Fixes #1009 (closed).

• tests: Fix import order of buildstream and tests modules

  Now that the buildstream module is inside src directory, pylint treats buildstream as a third party module inside tests. As such, it wants the imports from buildstream to be placed before imports from tests. Adopt this style to fix the lint errors, which is probably also good for readability.

• .coveragerc: Fix coverage reporting after move to src directory

  • include directive is unnecessary as all of buildstream is already included, so remove it.

  • Having buildstream/buildstream in paths confuses coverage combine as it tries to insert src/buildstream in the path twice in that case.