[GHC] #10622: Rename Backpack packages to units

[GHC]

#10622: Rename Backpack packages to units
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        Reporter:  ezyang            |                   Owner:  ezyang
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:  backpack
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D1057
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Comment (by ezyang):

 Duncan and I had a chat about this, which in particular clarified "What
 questions Cabal asks the installed package database."  cabal-install
 currently queries the installed package database in order so determine
 what the installed packages are when it is making an install plan.
 However, this is going to soon not be the case with Nix style versioning:
 Cabal will make an install plan using purely source package info, and then
 query the database to find out what products it already has installed.

 So the key idea is: **the installed package database is a database of
 units**!  There may be more junk in the database than you would expect
 from the packages you have installed, but in a Nix world where there will
 be many many installs of the same package, ghc-pkq is already going to not
 be so useful for finding info about packages.

 I'll update the top level description with the final plan.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10622#comment:8>
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