Make one more execution_server dep required

[vanja-p]

This is a follow up on #11251, which is possible after the changes in #11283.

[vanja-p]

PTAL. I had to pull out capabilities server from libmain.RegisterLocalServersAndClients. I'm not sure if the amount of refactoring is worth it.

[bduffany]

@vanja-p my interpretation is that RegisterLocalServersAndClients was kind of tangled up because it both registers the "base" dependencies needed by OSS + enterprise, but at the same time it has enterprise dependencies. So I think in that sense, it does seem worth breaking the circular dependency chain there, even though it's a little awkward to have to register the capabilities server in two different places.