[GHC] #8172: Expose CWD and import search paths in GHCi via new `:show paths` command

[GHC]

#8172: Expose CWD and import search paths in GHCi via new `:show paths` command
-------------------------------------------+-------------------------------
       Reporter:  hvr                      |             Owner:  hvr
           Type:  feature request          |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  GHCi                     |           Version:  7.7
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Easy (less than 1 hour)  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 - Currently, GHCi provides a `:cd` command but no respective builtin
 command to query the currently set path.
  - Moreover, there's no obvious way to query the currently active
 `importPaths` dynamic flags field, to find out which `-i` options are
 currently in effect.

 The attached patch extends `:show` with a `:show paths` subcommand,
 printing the CWD and the current value of `importPaths`:

 {{{
 Ok, modules loaded: Main, InteractiveUI, Paths_ghci_ng, GhciMonad,
 GhciTags.
 λ> :show paths
 current working directory:
   /home/hvr/Haskell/My/ghci-ng
 module import search paths:
   dist/build/ghci-ng/ghci-ng-tmp
   ghc
   dist/build/autogen

 λ> :set -i

 λ> :show paths
 current working directory:
   /home/hvr/Haskell/My/ghci-ng
 module import search paths: none

 > :cd /tmp
 Warning: changing directory causes all loaded modules to be unloaded,
 because the search path has changed.

 > :show paths
 current working directory:
   /tmp
 module import search paths: none
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8172>
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