Add metadata about conditionals to requirements

[tchoutri]

In binary.cabal:

library
  build-depends: base >= 4.5.0.0 && < 5, bytestring >= 0.10.4, containers, array
  if impl(ghc <= 7.6)
    -- prior to ghc-7.4 generics lived in ghc-prim
    build-depends: ghc-prim

And in the database:

SELECT r0.requirement
     , r0.components
FROM requirements AS r0
WHERE r0.package_component_id = '1b792ece-9da4-ba60-c309-02e1f181f1c6'
ORDER BY r0.components

+-----------------+----------------+
| requirement     | components     |
|-----------------+----------------|
| >=0             | ['array']      |
| >=4.5.0.0 && <5 | ['base']       |
| >=0.10.4        | ['bytestring'] |
| >=0             | ['containers'] |
| >=0             | ['ghc-prim']   |
+-----------------+----------------+

Obviously it would be interesting to know that ghc-prim is only depended on if GHC <= 7.6.

We store this information in the package component today:

SELECT *
FROM package_components AS p0
WHERE p0.release_id = '6ef7f226-c6b0-1a3b-17af-a70017193dff'

{"conditions": [{"tag": "Var", "contents": {"tag": "Impl", "contents": ["ghc", "<=7.6"]}}]

But this is not where this should go.