Enhance tools/scan_for_updates.py to also accept path to a PackageInfo.g file

[fingolfin]

For local testing before a release is made, it would be nice if one could write

 tools/scan_for_updates.py path/to/some/my_gap_package

and then it takes the PackageInfo.g in that directory (or complains if it is missing) and imports that instead of downloading anything.

Basically one could detect the case were there is a single argument that is a valid path to a directory containing a PackageInfo.g, and in that case skip most of the main function and directly call import_packages which takes a list of PackageInfo.g paths as argument anyway.