-package-db flags now need to be sorted by dependency order

After 39b71e81 putting the -package-db flags in the wrong order will result in a compile error:

<command line>: cannot satisfy -package-id b-1-XXX:
    b-1-XXX is unusable due to missing or recursive dependencies:
      a-1-XXX
    (use -v for more information)

See attached file for a repro.

It's reproduces in 8.0.1 and HEAD. Doesn't reproduce in GHC 7.10.3 or after reverting the above mentioned commit.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Package system
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ezyang
Operating system
Architecture