Package keys (for linking/type equality) separated from package IDs.

This patch set makes us no longer assume that a package key is a human
readable string, leaving Cabal free to "do whatever it wants" to allocate
keys; we'll look up the PackageId in the database to display to the user.
This also means we have a new level of qualifier decisions to make at the
package level, and rewriting some Safe Haskell error reporting code to DTRT.

Additionally, we adjust the build system to use a new ghc-cabal output
Make variable PACKAGE_KEY to determine library names and other things,
rather than concatenating PACKAGE/VERSION as before.

Adds a new `-this-package-key` flag to subsume the old, erroneously named
`-package-name` flag, and `-package-key` to select packages by package key.

RFC: The md5 hashes are pretty tough on the eye, as far as the file
system is concerned :(

ToDo: safePkg01 test is failing, but for a stupid reason: package deps now
line-wrap past the end, so the grep is not pulling everything.  So, we could
'make accept' the output, but it wouldn't be right!

ToDo: In a later commit, update all submodules to stop using -package-name
and use -this-package-key.  For now, we don't do it to avoid submodule
explosion.

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

Test Plan: validate

Reviewers: simonpj, simonmar, hvr, austin

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D80
Disable ghc-pkg accepting multiple package IDs (differing package keys) for now.

Duncan requested that ghc-pkg not accept duplicate package IDs (foo-0.1)
by default until the higher level tools can accommodate it.  Until then
you'll need to use the --multi-instance flag to install multiple copies in
the package database.

I think reusing the --multi-instance flag is dodgy, because that can be used
to cause duplicate package keys; but there is a mode of use of the database
where package keys are unique.

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