bootstrap.sh: alter check for tarballs already downloaded to work with symlinks also
support bootstrapping cabal-install into a sandbox
Merge pull request #2137 from JakeWheat/bootstrap-sandbox

'support sandboxes' in bootstrap
Whitespace.
80-col violations.
Switch InstallPlan over to using IPID-indexed PackageIndex.

While this doesn't let us get rid of Distribution.Client.PackageIndex,
it does make InstallPlan more flexible, so we can have the same
package name-package version in the install plan multiple times.
We do this by synthesizing fake installed package IDs to act
as placeholders prior to compilation.

There is some shindig with 'FakeMap' in PackageIndex, check out
the Note in that file for more details.

This reverts commit a5a0f8e1959003ee702c04a23375a60d48f03f90, with
a bugfix for linearizeInstallPlan.

Fixes #2123
Better diagnostic information for install plans.

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