fix(findcode): better finding of test code

This fix sorts the --paths to look for code.
Longest paths come first. This is for cases where
one checks out other sources within his source directory.
In such a case this structure can happen:
my_package
my_package/src/some_other_package

Now when my_package gets found first. It logs an import
error and then goes on.
by sorting with longest paths first. I always get the most
specific match first. Since the loop is short circuited here,
it will never look at my_package.

This fixes #15
Suggest rephrasing as

    sorted_prefixes = sorted(options.prefix, reverse=True, key=lambda p: len(p[0]))

Also I wonder if it makes more sense to sort by `len(os.path.abspath(p[0]))` instead?