- Temporary work directories created for modules whose compilation process
  does not work properly (e.g. because there is no associated C code) are now
  deleted immediately instead of waiting until the process ends. Note that we
  could probably make it so that we do not create then delete too many
  directories by checking if the C implementation works before creating the
  directory.
- KeyData objects associated to unversioned modules are now created, so that
  the module can be re-used by another unversioned key in the same process.
  They are not saved though.
- Versioned but broken keys can now also re-use modules.
- The version part of unversioned keys is now ignored in the module hash, just
  in case there would be multiple unversioned keys whose string representation
  of the version would be different.
- Added a 'debug_counter' utility function that can be used for debugging
  (may be moved later to a more generic place if someone wants to re-use it
  in another module)

Test different cases of error and normal behaviour.

It enables using that code in scan.