import: pypi: Detect inputs.

* guix/import/pypi.scm (python->package-name, maybe-inputs, compute-inputs,
  guess-requirements): New procedures.
* guix/import/pypi.scm (guix-hash-url): Now takes a filename instead of an
  URL as input.
* guix/import/pypi.scm (make-pypi-sexp): Now tries to generate the inputs
  automagically.
* tests/pypi.scm: Update the test.