references: Refactor to only use lconf.ImportPkgs
references: Refactor to only use lconf.ImportPkgs

This is a no-op change to make it clearer how the next change works. We had some
weird interactions between collectPkg and lconf.ImportPkgs and AfterTypeCheck.
This moves as much of the expected contract to findReferences, as well as making
the next changes clearer.

sourcegraph/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver,lambdalab/go-langserver