Macros want to register usage of import

[som-snytt]

Scala 2 macros don't have sanctioned API to mark an import as "used" wrt -Xlint.

Presumably, a macro uses inferImplicit (which doesn't mark the import as used because implicit searches probe the implicit scope multiple times) but then wants to lookupSymbol explicitly to say "yes, I used this one."

That may be possible but not convenient.

[som-snytt]

Also other symbols: #10313