In the source tarball we don't want to include `.configured` files so
that a build will call `npm i` first.
Fixes: #15905
Follows: #15926
Test-bot: skip
This change adds a new GitHub action that verifies that Keyman can build
from the source package. Also check that source package used for Launchpad
and Debian/Ubuntu packaging doesn't produce lintian errors. Previously
lintian errors did go unnoticed because they didn't stop the build.
Also refactor `launchpad.sh` script to be a proper builder script, and add
some new parameters to allow skipping some steps in the script.
Build-bot: skip
Test-bot: skip
