Allow unqualified component syntax

[mgsloan]

Currently, package components need to be qualified by PKG:test:TEST_NAME / etc. However, when I was poking around the Cabal code I noticed that having components with different types but the same name is no longer supported. Since it will be unambiguous, we should allow PKG:TEST_NAME. One question is if you write PKG:test, to refer to a component called test should that be allowed? I think so.

I actually think it should have been this way even if Cabal did not enforce this constraint. If it is aliased, just build both.

Similarly, I would like to support :component-name syntax. This would build all the components in the local packages matching that name. We could even consider glob patterns, but that is getting a little wild maybe.

[mitchellwrosen]

However, when I was poking around the Cabal code I noticed that having components with different types but the same name is no longer supported.

Do you remember where in the code you saw this? It doesn't sound right to me (and the docs at http://cabal.readthedocs.io/en/latest/nix-local-build.html#cabal-new-build indicate otherwise, though I know they're always going out of date).

[mgsloan]

@mitchellwrosen Here https://github.com/haskell/cabal/blob/fix-travis-stack-build/Cabal/Distribution/PackageDescription/Check.hs#L200 . Not 100% sure, since it's part of cabal check. It does use the word "must", but maybe it is not actually required..