Fixes #1901. Some variants of 'tar' don't support the '--format' option, so we
have to do feature detection.

Note that 'cabal sdist' (as opposed to 'Setup.hs sdist') uses the built-in
tar.gz archiver ('Distribution.Client.Tar') and doesn't invoke 'tar' or 'gzip'.

∙ At one point under OS X, ld required the mtime of the symbol table to be
  not older than that of the archive itself. This means that even copying an
  .a file required invoking ranlib to update the symbol table mtime.
∙ However running ranlib renders pointless the deterministic mode of #1537.
∙ As of at least 2008 according to the BUGS section of ranlib(1), this has
  no longer been necessary.

First introduction: 12b35902
Current implementation: af3da760

Previously we would just use the normal system $PATH for finding
programs (unless the Program provided some custom method). Now the
ProgramDb has its own notion of the search path and we use that for
finding programs (by default). The search path can be either specific
directories or the system search method (ie $PATH on unix and something
similar on Win32). The default search path is just the system one.

In addition, this search path is passed on to programs when we invoke
them as the $PATH env var.

Makes things a little simpler in GHC's build system if libraries are in
the same directory as their name.

This patch also reorganizes the HPC output directories for consistency. All
files related to HPC are now located in the "dist/hpc" directory.

At the top level we now have cabal/ and cabal-install/

Break a couple fewer package's Setup.hs files

Add an explicit intermediate ProgramInvocation data type.