This allows us to compile Setup.hs without depending on base.
In particular, this ensures that a package with `build-type: Hooks`
and a custom setup stanza that does not depend on base successfully
compiles.

Tested in PackageTests/SetupHooks/SetupHooksNoBase.

This commit introduces a new build-type, Hooks. A package using this
build type should provide a SetupHooks.hs module which exports
a value `setupHooks :: SetupHooks`. This is intended to replace the
Custom setup type. This allows Cabal to have finer-grained information
about the build, instead of having an opaque Setup executable to invoke.

Tests include:

  - error when returning an invalid component diff in a
    per-component pre-configure hook
  - error when declaring pre-build rules whose dependency
    graph contains cycles
  - error when we cannot find a dependency of a pre-build rule
  - warning when there are pre-build rules that are declared but
    never demanded
  - correctness tests for SetupHooks, e.g. that
    pre-build rules are run in dependency order (see the
    `SetupHooksRuleOrdering` test)

This commit modifies the generation of runghc commands to pass an
additional -i argument to account for the change in working directory.

This ensures that, in the testsuite, runghc is able to see other modules.
For example, an invocation of runghc to run a Custom Setup script will
now properly see the modules imported, e.g. if one has a directory
structure like

  test.cabal
  Setup.hs
  SetupDep.hs

and Setup.hs imports SetupDep.hs.