make BuildStream License Checker and official BuildStream project

[Douglas Winship]

Hi everyone.

For a couple of weeks, we've been working on a Buildstream license 
checker script. We're hoping that it can become a useful tool for the 
BuildStream community, and we'd like to bring it under the BuildStream 
umbrella by making it a project in the BuildStream GitLab group.

The tool is a python script that interacts with a BuildStream project by 
invoking BuildStream commands like 'bst show' and 'bst workspace open' 
(using subprocess.call). The script checks out the source code of 
BuildStream elements into temporary folders, and then uses a separate 
piece of software called licensecheck to scan the files for license 
information.

The final output is a list of licensecheck output files (one for each 
element), plus human readable and machine readable summary files (html 
and json, respectively). We're currently testing the tool by running it 
in CI on a Freedesktop SDK branch. Sample outputs can be seen in the CI 
job artifacts.

We'd appreciate feedback on the script itself, and on the idea of making 
it part of the BuildStream group.

Douglas Winship

Links:

GitLab repository for the license checker tool: 
https://gitlab.com/DouglasWinship/buildstream-licence-checker
The license scanning software used by the tool: 
https://metacpan.org/pod/distribution/App-Licensecheck/bin/licensecheck
Freedesktop-sdk MR, using the tool. (See CI job artifacts for sample 
outputs): 
https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/merge_requests/3293