Be more lazy in namedImportRef
Be more lazy in namedImportRef

We got a hard to track down error when changing to the new
annotations elaboration scheme (should be in the next commit):

When running `testNonCyclic`, `DotClass` was not found in object Trees
even though it was imported from `util`. It turned out that the import
was ignored because the `util` symbol was completing.

This commit adds a warning when this happens. The warning currently applies
only to named imports because several false negatives were encountered if
we do this also on wildcard imports. I.e. we get a warning, but the searched
after symbol is not a member of the wildcard qualifier.

This commit also refactors namedImportRef, so that `site` is only
computed when the name to reseolve appears in the selector list.
That change made the previously observed error go away because less
is now forced.

som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty