This is used in _fdroiddata_'s `fdroid build` CI job, and _issuebot_.
This uses @uniqx's awesome new 'plugin' feature to create a 'fetchsrclibs`
command. The 'fdroid build' gitlab-ci job uses --on-server, which does not
set up the srclibs. This plugin does the missing setup.
This is moved from _fdroiddata_ where it was _tools/fdroid_fetchsrclibs.py_.
