Merge: contrib: apply nitpackage --gen-ini

Merge: contrib: apply nitpackage --gen-ini

I ran `nitpackage --gen-ini` on contrib projects.

First of all, I added 3 more accepted keys to nitpackage --check-ini that seem to be used in some projects (04abe90):
* `upstream.apk`
* `upstream.tryit`
* `source.exclude`

I expanded the package `github_merge`, the only one left in the non-expanded form in the Nit project (51df98b).

Then I corrected some `git.directory` keys with incorrect values (a4b53f7). Short note for `src`, I added the trailing `/` to be consistent with all other ini files.

Finally I generated the `package.desc` for all projects from the `mdoc_or_fallback.synopsis` values (051ab50).

Pull-Request: #2682
Reviewed-by: Jean Privat <jean@pryen.org>