Don't solve for executables in legacy code path.
There is a bug in `cabal configure`'s invocation of the solver in Distribution/Client/Configure.hs: standardInstallPolicy installedPkgIndex (SourcePackageDb mempty packagePrefs) [SpecificSourcePackage localPkg] We can see that the solver is given an EMPTY source package database. This is because we assume that everything you need from cabal configure is taken from the installed package index. But this is NOT true for executables, which never have an entry in the installed package index. So we SHOULD NOT solve for executables in the legacy codepath, since there isn't anything useful we can do with the info anyway. This gets toggled using a new solver parameter SolveExecutables. I didn't bother with a test because this will be obsoleted by nix-local-build. Fixes #3875 Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>