[GHC] #9606: Improve outdated ghc-pkg cache warning

[GHC]

#9606: Improve outdated ghc-pkg cache warning
-------------------------------------+-------------------------------------
       Reporter:  nh2                |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  ghc-pkg            |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 Sometimes you get this:

 {{{
 WARNING: cache is out of date:
 /home/user/.ghc/x86_64-linux-7.8.3/package.conf.d/package.cache.
 Use 'ghc-pkg recache' to fix
 }}}

 The warning is correct but a bit confusing, because running `ghc-pkg
 recache` does nothing here: It will by default update your global package
 DB, and the DB the warning is about will still be out of date.

 This seems to have frustrated others as well:
 https://gist.github.com/mcandre/5789498

 I suggest we make the message more clear: `You might have to pass --user
 to update this cache file`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9606>
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