bug#36084: ghc-tasty/ghc-clock circular dependency breaking is broken

[Robert Vollmert]

ghc-tasty depends via “inputs” on ghc-clock-bootstrap (v0.5) which is built 
without tests,
while ghc-clock (v0.7) depends via “native-inputs” on ghc-tasty for tests.

This means that any package which depends on ghc-tasty and ghc-clock is 
potentially broken,
e.g.:

Warning:
    This package indirectly depends on multiple versions of the same package. 
This is very likely to cause a compile failure.
      package tasty (tasty-1.1.0.3-98rSghuW4l26JGzzQLN3ha) requires 
clock-0.5.1-KAiVgaxjUlIIuEB7RoOIe6
      package hspec-core (hspec-core-2.5.5-BSfG8Pnx1al9rTpu1j0PaW) requires 
clock-0.7.2-JStwYFlKVmNHl0K1ll1Mx5

I’d suggest breaking the cycle instead by

1. introducing tasty-bootstrap which builds against the `time` module via the 
cabal flags:

flag clock
  description:
    Depend on the clock package for more accurate time measurement
  default: True

This could be done via
  (arguments `(#:configure-flags (“—flag=-clock”)))
as far as I understand.

2. changing ghc-clock to test via tasty-bootstrap (and possibly some more 
variant testing
packages that would be changed to depend on tasty-bootstrap)

3. changing tasty to test via ghc-clock.


(I gave this approach a shot myself, but I’m running into mysterious silently 
hanging guild and guix build
processes — possibly some cyclic dependencies which are noticed?)