If the coverage.py source file is not found in the source directory, the
build directory is first searched before raising an error.
This is necessary because it is a valid workflow to build a Python
package from source, then install this package to a virtualenv that
lives in the build directory. Tests will run against this deployed
package and therefore the covered source files will be found in a
subdirectory of the build directory, and not anywhere in the source
directory.
This assumes that coverage.py has been run in such a way to produce its
standard XML output. This uses the Cobertura schema and should be somewhat
generalizable.