[GHC] #15145: Emit info-level log message when package environments have been picked up

[GHC]

#15145: Emit info-level log message when package environments have been picked up
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           Reporter:  hvr            |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  high           |         Milestone:  8.4.3
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 A common complaint with the new package environment files feature is that
 it's not obvious when package environments have be picked up. Even though
 this feature has been in GHC since GHC 8.0, now with cabal 2.2 turning on
 ghc env file generation, this getting wider exposure.

 A similiar UI issue existed with GHCi and its `.ghci` files and the
 solution there was to emit a message informing the user that an implicit
 configuration file was picked up:

 {{{
 GHCi, version 8.5.20180512: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/hvr/.ghci
 Prelude>
 }}}

 The same solution would greatly reduce the confusion with package
 environment files which have an even trickier lookup logic than `.ghci`
 files.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15145>
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