Pull PackageDescription from readDesc

[thumphries]

Overriding readDesc doesn't quite work; there are a few spots that ignore the stashed value and read the .cabal file again from disk, notably cleanAction and sdistAction. This means defaultMainNoRead doesn't work properly for clean or sdist.

• Respect readDesc when it is defined, instead of re-reading the cabal file from disk
• Provide defaultMainWithHooksNoRead shorthand, this is helpful

[thumphries]

Apologies for submitting this PR prematurely, I was having trouble testing (Setup.hs kept being run against the system Cabal). If there's anything I can do to improve it, please let me know.

[ezyang]

LGTM.

But perhaps readPackageDescription deserves a comment telling you to look at readDesc when you invoke it.

[ezyang]

Merged.