Let qtattributionsscanner get licenses from LICENSES directory

Now that Qt sources ship explicit licenses in LICENSES/*.txt, qtattributionsscanner could use these licenses instead of an explicit "LIcenseFile" entry too. This would allow us to avoid multiple literal copies of say the GPL.

Implementation proposal

If a qt_attributions.json entry misses LicenseFile or LicenseFiles information, qtattributionsscanner walks up the hierarchy for a directory called LICENSES. If it finds one, it checks whether there are files matching all existing entries in the LicenseId entry. If it finds all licenses listed there, it uses the files from there.

In case only some or no licenses are found, an error is printed.