Improve process to publish/consume m2e-wtp

m2e gets pushed to the updates/requirements/m2e-wtp.juno url, whose content get overridden. However, published target-platform still depends on old content until they get modified.

We should find a better process that prevent target platforms from being broken, and that would make build reproducible.
A solution would be to keep old versions of m2e-wtp under the m2e-wtp.juno folder (1 child folder for each version), and to modify target-platform on request to use the newer version.