[GHC] #15439: Refactor `printMinimalImports` into two functions and reexport them

[GHC]

#15439: Refactor `printMinimalImports` into two functions and reexport them
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           Reporter:  chshersh       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  GHC API        |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
  imports,refactoring,source         |
  plugins                            |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Currently the `printMinimalImports` function finds set of minimal imports
 and prints them into file.

 *
 https://github.com/ghc/ghc/blob/0f5a63e3d763f18c683f076e0e96396166863f56/compiler/rename/RnNames.hs#L1469

 But it's more convenient to have function that returns set of minimal
 imports that can later be used by external tools like source plugins
 without need to parse content of `.import` file. And without need to copy-
 paste existing function.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15439>
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