cabal new-build lib / pkgname:lib should build library

[ezyang]

Requested here https://www.reddit.com/r/haskell/comments/50qr05/documentation_for_cabal_newbuild_feedback/d76wb7u?context=3 . At the very least pkgname:lib would be welcome.

I'm less certain about lib: it is probably ambiguous in a multi-package project. I don't know if the behavior should be, "if you are in the working directory of a Cabal package, lib refers to the specific library of that Cabal package"

This means lib needs to be reserved see #3746