Add --coverage-only-library, make default coverage behavior simpler

[mgsloan]

I'm revisiting #634 ("stack test --coverage shouldn't include tests in reports")

• It's causing issues on 7.10 - stack test --coverage fails sporadically #1181
• It's common for packages to define their tests in such a way that they end up building the library directly with the test.

In light of this, I think it's a good idea to put this behavior behind a flag like --coverage-only-library.

[mgsloan]

Pinging @mboes, does this seem reasonable?

[mboes]

Fair enough. But I assume that, if enabled, one is likely to run into #1181?

[mgsloan]

I've decided not to do this. This stuff got a bit complicated, I don't expect this to be 100% clear:

I've reinstated my old solution to #1181 with a fix for #1162, so actually I think things should be fine now, and this hopefully isn't "causing issues" any longer.

Also, when I tried out implementing this, but ran into issues with cabal files that have multiple executables, due to https://ghc.haskell.org/trac/ghc/ticket/1853#comment:23 . Since I can't use the hpc program's --include argument to leave out the unqualified modules when there isn't a library stanza (#1162), my solution for now is to munge all tix files to remove unqualified modules.

Since I'm removing all unqualified modules, we don't really get the benefit of having a solution for packages that build their library code directly into their tests.