Enable "legacy" profile in Extension Maven handler

The idea is that you usually want to install the deprecated APIs when you install an extension with extension manager (for example the XWiki Standard flavor has legacy dependencies containing some APIs). But the rule is generally to put legacy dependencies in a profile so that it's possible to build without them, in which case EM won't see them.

The fix for that is to always activate profile with name "legacy" in EM for now and introduce a configurable switch later if needed.