Error spans of unused imports could be improved

Example:

compiler/codeGen/StgCmmExpr.hs:44:1: warning: [-Wunused-imports]
    The import of ‘primRepSlot’ from module ‘RepType’ is redundant
   |
44 | import RepType          ( isVoidTy, countConRepArgs, primRepSlot )
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

compiler/codeGen/StgCmmExpr.hs:53:1: warning: [-Wunused-imports]
    The import of ‘Data.Function’ is redundant
      except perhaps to import instances from ‘Data.Function’
    To import instances alone, use: import Data.Function()
   |
53 | import Data.Function ( on )
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Second error message is fine, but in the first one the span looks as if the whole import line is the problem. I think a better error would be:

compiler/codeGen/StgCmmExpr.hs:44:54: warning: [-Wunused-imports]
    The import of ‘primRepSlot’ from module ‘RepType’ is redundant
   |
44 | import RepType          ( isVoidTy, countConRepArgs, primRepSlot )
   |                                                      ^^^^^^^^^^^

Now it's clear that only the primRepSlot part is the problem.

(Note that in the "improved" error message I also updated the "column" part of the location)