[clangd] Rename Module -> FeatureModule to avoid confusion. NFC

As pointed out in D96244, "Module" is already pretty overloaded to refer
to clang and llvm modules. (And clangd deals directly with the former).

FeatureModule is a bit of a mouthful but it's pretty self-descriptive.
I think it might be better than "Component" which doesn't really capture
the "common interface" aspect - it's IMO confusing to refer to
"components" but exclude CDB for example.

Differential Revision: https://reviews.llvm.org/D97950