Only build needed executables in a package

[Ericson2314]

Once #4055 is merged, build-tool-depends will be implemented, but somewhat shoddily in that on the cabal-install side the exact needed exe dependency will be thrown away and all built instead. This is extra unneeded work, and also changes the PATH in potentially unexpected ways.

Additionally, #3708, the original issue, proposed there also be wildcard deps, for depending on all exes in the package as the implementation currently does, which #4055 did not implement. Parsing and handling those would be a good thing to do in conjunction with this, lest the wildcard case in solving and building be deleted and then reimplemented.

[Ericson2314]

See #4104 (comment) for a roadmap. Thanks @ezyang !

[Ericson2314]

The original issue proposed plain $package for depending on all exes in a package, but I'd prefer $package:*. Then, if named library components are ever exposed (#4206), one could analogously do $package:* in build-depends to depend on all libraries in a package. Further keeping the syntax analogous, we should reserve $package for packages gaining a notion of a primary, unnamed exe component.