This fixes Lintian warnings and other comments from the Debian review of the package. It also removes unnecessary files from the source package.
Note: this PR uses `Expat` instead of `MIT` in the `debian/license` file since that is the term that Debian uses and that the Debian tools output. It doesn't really matter since the term serves only as a key in the `license` file that links the files to the full text of the license further down in the `license` file.
Fixes: #13905
Previously the builder scripts defined a readonly `VERSION` environment
variable for the Keyman version. That caused problems when another
(external) script tried to define a `VERSION` variable. We encountered
this problem when trying to move the TC build steps of a configuration
into a single script (#13399) when we tried to source `~/.nvm/nvm.sh`.
This change uses a Keyman specific prefix for the version variables and
renames `VERSION` → `KEYMAN_VERSION` etc. Unfortunately these variables
are used in a lot of places, so this turned out to be a bit of a yak
shave.
Test-bot: skip
