Fix --program-{prefix,suffix} resulting in invalid installation

Currently the options `--program-{prefix,suffix}` for cabal install
affects the name of the file in the install directory *and* the
executable name in the store. The installation fails:

- If using `--install-method=symlink`, the *target* of the symlink is
  not affected by the affix options and it results in an invalid symlink.
- If using `--install-method=copy`, the copy fails because the source
  is not found.

Another issue is that it affects the computation of the hash of the
build directory in the store, resulting in needless rebuild when using
successively different affix options.

Fixed by making the name of the executable in the store canonical, i.e.
always ignoring the program affix options.

Added a test for all the combinations of `--install-method` and program
affixes options.