[RFC] Negative specification of cabal.project fields affecting package resolution

In certain scenarios, where:

1. We have magical knowledge that the GHC's package set is sufficient to build the project, and
2. We still need the cabal.project for some extra information (flags, relative package locations etc)

We might want to:
1. suppress the aspects of project file's specification that affect package resolution,
2. and further, being able to express that suppression via `cabal.project.local`

Hence, I propose that we add two new directives to cabal project files:

1. `ignore-source-repositories`, that would have the effect of cancelling all effects of `source-repository-package` declarations
2. `ignore-constraints`, to cancel all effects of `constraints` declarations.

cc @dcoutts