Variant of source_mapping that maps prefixes, not files #30

Background

source_mapping basically lists out every file that ought to be included when building a wheel, giving source and dest name. For example,

setup(
  package_dir = "src"
  packages = ["a", "b"],
  py_modules = ["c"],
)

might return

{
  "a/__init__.py": "src/a/__init__.py",
  "b/__init__.py": "src/b/__init__.py",
  "c.py": "src/c.py",
}

This task

Make a new source_mapping_prefix method that just gives back more like

{
  "a": "src/a",
  "b": "src/b",
  "c.py": "src/c.py",
}

Alternatively you could have a dict of prefix to list of tuple of item src/dest, but the example above is probably easier. Most of the effort is in coming up with good test data, and making this work for all the backends.