2.13.2 regression: enabling -Ymacro-annotations defangs -Wunused:imports

[SethTisue]

% cat S.scala                                           
import java.util.List
% scalac --scala-version 2.13.2 -Wunused:imports S.scala
S.scala:1: warning: Unused import
import java.util.List
                 ^
1 warning
% scalac --scala-version 2.13.2 -Wunused:imports -Ymacro-annotations S.scala
%

the import is unused, but adding -Ymacro-annotations prevents that from being reported

this regressed in 2.13.2; if you do the above on 2.13.1 it's fine

(reported by @Billzabob on Gitter; great catch!)

(note that --scala-version comes from scala-runners, though that's not important to the bug)

[SethTisue]

attn @som-snytt @lrytz

[som-snytt]

Probably because Namers must makeImportContext after scala/scala#8801