Do all-in-one package builds when there are no cyclic dependencies

[snoyberg]

Discussed with @mgsloan already. The current reason for this is dealing with code coverage better, but this should in general make Stack behave better for common cases. (@meteficha, you in particular will probably like this.)

Currently, Stack does a separate cabal build for the library/executables, tests, and benchmarks. The reason is cyclic dependencies: package B may depend on package A, while package A's test suite depends on package B. We want to tweak this as follows:

1. When there are cyclic dependencies: act just as we do today
2. Otherwise: force all test and benchmark dependencies (as requested) to be built before building a package, and then build all necessary components at once.

[meteficha]

Nice! Just to make sure I understood the implications, the advantages are that:

• There will be fewer dependencies between packages, thus more opportunity for parallelism.
• Fewer cabal builds per package will mean less overhead.

Is that correct?

[snoyberg]

I don't know that (1) will happen; actually, the opposite is likely true, though I'm not certain. The second is the real advantage (and avoiding some unfortunately Cabal cases with unnecessarily-overwritten cabal_macros.h).

[mgsloan]

I've resolved #1181 with a different approach, so this isn't so urgent. It would be a nice optimization, though.

Almost humorously, the only way to work around a different --coverage issue is to build tests one at a time - https://ghc.haskell.org/trac/ghc/ticket/1853#comment:23 (tl;dr; there are problems getting coverage for modules built directly into an executable, when there are multiple executable stanzas).

[mgsloan]

EDIT: long outdated comment removed. Old text: https://gist.github.com/mgsloan/f2c671cc7ca9f4ecbd07

Other notes:

• This may also allow us to address this TODO
• @borsboom mentioned that it seems like there are redundant uses of writeBuildCache. I think this would help consolidate that logic and be able to remove the redundant uses.