Add module context to all declarations. As keeping the `std::variant`s
required slightly more work for this, and we intended to remove them
any way, this is done in this change.
For background, we put those in as we were not extracting all modules at
the time, so we were missing some DB ids that we put back in by adding
the "defining" traps not only in the trap file related to the defining
module but also to the one where the declaration was used. Since then
we fixed module extraction, so this should not be needed any more.
Deduplication of `ConcreteVarDecl` is triggered only if its
`DeclContext` is not local. This avoids a mangled name conflict.
Also added more thourough tests for `ConcreteVarDecl` and `ParamDecl`.
