Dependencies are now managed by Composer

Some tools could be managed in their own repositories, as they could also be
usaed as standalone products.

Previously, Git submodules where considered to take care of this scenario, but
we removed this support in 9db7d7097b93b0c74dfd7e8b4c4f36e8bacbc962 as that's
not flexible enough and would provide extraneous commits for each submodules'
last relevant revision.

Instead, dependencies are now managed by Composer and a Makefile is responsible
to move tools at the right place. Libraries could stay in the vendor/ directory.

First tool migrated is wikimedia/write/sourcetemplatesgenerator/