modify qtqa's tst_license.pl to accept SPDX licenses

Modify qtqa's tst_license.pl to accept SPDX licenses for repositories that have been ported to the REUSE specification.

One way to achieve this is to modify tst_license.pl to scan for SPDX license identifiers and verify the existence of files in the LICENSES directory.

That would be a re-implementation of the reuse linter.

It would be nicer to directly use "reuse lint".

Challenges:

• We need to provision reuse to CI. This adds a dependency to pypi.org. Alternatively we can host the reuse wheel on our download server.
• We need to be fully compliant to the REUSE spec if we want to use "reuse lint". Or we need to check the output of "reuse lint" and ignore known issues.

Problems with full compliance are:

• Adding foo.png.license to every binary file foo.png
• 3rdparty libraries