LocalBuildInfo should change at build time when components are selected

You can specify which components to build like this: `runhaskell Setup.hs build my-testsuite`. But since `LocalBuildInfo` is generated at configure time, the `componentsConfigs` field list every component that was enabled then. This means you can't check whether certain components are being built.

I ran into this trying to update my PR #2844. `cabal-install`'s `Setup.hs` builds the manpage in a `postBuild` hook. If we're haven't built `cabal` yet and `dist/build/cabal` doesn't exist, then the `openFile` call to write `dist/build/cabal/cabal.1` will fail. We need to only try to make the manpage when building the executable component but it's impossible to find out at this stage.