Fix defaut top level targets

[snowleopard]

This does the following:

• Fix the top-level targets, which appear to have been restricted to Stage1 targets only in Use Cabal directly in place of ghc-cabal + make build root configurable #531.
• Restore the stage1Only logic, remove outdated comments.
• Build touchy in Stage1, as requested in Build with --quickest didn't produce stage1/touchy #653.
• Move ghcTags and haddock packages to Stage2, as in the Make build system (Follow-up to #531 #540).
• Keep only Stage0 and Stage1 package databases (we do not need Stage2/Stage3 package databases).

[alpmestan]

Does this mean cross-compilers don't get their ghc-tags (cross-compilers are stage 1 GHCs) anymore ?

[snowleopard]

@alpmestan According to this commit ghctags should be built with Stage2 compiler.

[snowleopard]

Also, this note seems relevant: Note [No stage2 packages when CrossCompiling or Stage1Only].

https://github.com/ghc/ghc/blob/3bdf0d01ff47977830ada30ce85f174098486e23/ghc.mk#L1499-L1546

[alpmestan]

Oh, nevermind then!

[snowleopard]

Actually, there is this bit:

# Here's why:
#  - first of all, ghc-stage1 can't use stage0's ghc library (it's too old)
#  - neither do we register the ghc library (compiler/stage1) that we build
#    with stage0. TODO Why not? We do build it...
#  - as a result, we need to a) use ghc-stage2 to build packages that depend on
#    the ghc library (e.g. ghctags [4]) and b) exclude those packages when
#    ghc-stage2 is not available.

See the TODO -- I believe now we do register the ghc library built in Stage0, so presumably this means we could indeed now build all its dependencies in Stage1. Let me create a separate issue to discuss this.

[snowleopard]

Here is the issue: #661.