Stack builds packages when called with `--only-dependencies` in some multi-package projects

[rjmk]

General summary/comments

If you have a project with packages a and b, and b depends on a, stack build --only-dependencies will build a.

Bisecting, this change was introduced in d1e6c48. I'm not sure if it was introduced intentionally or not. If it's unclear why some might prefer this behaviours absence, consider building pedantically. You don't want to to be responsible for warnings in all your dependencies, so would rather build deps first unpedantically, and then be able to build all your own packages pedantically.

Steps to reproduce

1. Make a stack project with two packages in sub-directories.
2. Have one package depend on the other.
3. stack build --only-dependencies

Expected

I expect neither of the packages to build

Actual

The depended-on package builds

Stack version

$ stack --version
Version 1.5.1, Git revision 600c1f01435a10d127938709556c1682ecfd694e (4861 commits) x86_64 hpack-0.17.1

[rjmk]

I've created a pull request in case this change was not intended (I didn't see it in the CHANGELOG)

[mgsloan]

Merged, thanks!