Muddle init can add spurious "src" to build description path.

[dave-kynesim]

In a directory with an existing git repository, running:

muddle init weld+ssh://foo src/builds/01.py

where src/builds/01.py is the genuine path to the build description, causes a failure where muddle apparently looks for the description at src/src/builds/01.py.

Omitting the src, so that when muddle adds it the correct path results, works, but is somewhat counter-intuitive.