Cabal should include build targets in `--list-options`

This would simplify implementing [`(bash)Programmable Completion`](http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion.html) for commands taking a build target, such as `cabal build`, `cabal test` or `cabal run`, as the current state would require to reinvent `cabal`'s strategy for locating the `.cabal` file and parsing the cabal file for potential targets (while not being able to detect currently disabled targets)

So basically this just requires to include the targets where applicable in the `--list-options` output.

Start by locating the Cabal CLI handling for --list-options and the target discovery used by commands such as cabal build, cabal test, and cabal run. Trace how the .cabal file and disabled targets are handled, then verify that applicable build targets appear in the --list-options output.

Written by the indexing model from the issue text.