Allow limiting the scope of '--allow-newer'.

[23Skidoo]

Fixes #2756.

[23Skidoo]

/cc @dcoutts @hvr

[hvr]

@23Skidoo 👍

one question though, does the allow-newer field only allow True/False? @dcoutts: as the new cabal.project features allows per-package flags; it'd be neat if we could then write something like

packages: myproject.cabal, somemodded-pkg/*.cabal
-- applies to all
with-compiler: /opt/ghc/8.0.1/bin/ghc
-- corresponds to `--allow-newer: ghc-prim`
allow-newer: ghc-prim
tests: False

-- corresponds to `--allow-never aeson:base,aeson:process`
package aeson
   allow-newer: base,process

[23Skidoo]

@hvr

one question though, does the allow-newer field only allow True/False?

Yes, but I can change it to also support the full syntax if you want.

[23Skidoo]

Yes, but I can change it to also support the full syntax if you want.

Done.