Motivation
Suppose a user already has coverage as a dep, but we want to install coverage[toml] for pyprojec.toml integration.

I think that at the moment, this wouldn't work because we would see coverage is already a dep. But we really need to force the extra to get added to whatever other extras might be there.

Summary of desired enhancement
We should force extras to be installed - even if the base package is already installed without extras or with different extras.
Of course testing for this would be good, and the coverage package is the case in point.