Thinning and renaming modules from packages on the command line.

Summary:
This patch set adds support for extra syntax on -package and related
arguments which allow you to thin and rename modules from a package.
For example, this argument:

    -package "base (Data.Bool as Bam, Data.List)"

adds two more modules into scope, Bam and Data.List, without adding
any of base's other modules to scope.

These flags are additive: so, for example, saying:

    -hide-all-packages -package base -package "base (Data.Bool as Bam)"

will provide both the normal bindings for modules in base, as well as
the module Bam.

There is also a new debug flag -ddump-mod-map which prints the state
of the module mapping database.  H = hidden, E = exposed (so for
example EH says the module in question is exported, but in a hidden
package.)

Module suggestions have been minorly overhauled to work better with reexports:
if you have -package "base (Data.Bool as Bam)" and mispell Bam, GHC
will suggest "Did you mean Bam (defined via package flags to be
base:Data.Bool)"; and generally you will get more accurate information.
Also, fix a bug where we suggest the -package flag when we really need
the -package-key flag.

NB: The renaming afforded here does *not* affect what wired in
symbols GHC generates.  (But it does affect implicit prelude!)

ToDo: add 'hiding' functionality, to make it easier to support the alternative
prelude use-case.

ToDo: Cabal support

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: new tests and validate

Reviewers: simonpj, simonmar, hvr, austin

Subscribers: simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D113

GHC Trac Issues: #9375

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