cabal new-build [all] fails to honour --dependency-only

[hvr]

#3815 implemented --dependency-only handling, but didn't yet properly handle the implicit build-all cabal new-build --dep case as was pointed out in #3815 (comment) (relevant part quoted below).

This can be workarounded for now by using --dependency-only with explicitly named targets rather than the implicit "all" target.

---

@dcoutts This seems to work nice, except for the cabal new-build case when using a cabal.project file (and no .cabal file in the top-level folder & with CWD=top-level-folder) and thus causing it to implicitly build all packages: in this case cabal new-build --deps seems to consider nothing to be a dependency goal, e.g.

$ cd /tmp/foo
$ cabal get uhttpc-0.1.1.0
Unpacking to uhttpc-0.1.1.0/
$ echo "packages: uhttpc-0.1.1.0"  > cabal.project
$ cabal new-build --dry
Resolving dependencies...
In order, the following would be built (use -v for more details):
 - bytestring-lexing-0.5.0.2 (lib:bytestring-lexing, lib:bytestring-lexing) (requires build)
 - uhttpc-0.1.1.0 (lib) (first run)
 - uhttpc-0.1.1.0 (exe:uhttpc-bench) (first run)
$ cabal new-build --dep
Up to date