allow cabal.project to specify/override additional pkg-config depends

so i'm trying to use the rounded package on recent ghc, and ignoring other challenges, I need to make it aware of a gmp install, because recent ghc dont provide a gmp!

```
packages: *.cabal

allow-newer: rounded:base,rounded:ghc-prim,long-double:base

package rounded
pkgconfig-depends: mpfr, gmp
```
is what i'm trying to do, but it looks like that field isn't understood by cabal.project?

Start by examining how cabal.project package stanzas handle fields, using the shown pkgconfig-depends setting as the concrete case. Compare that behavior with the package's .cabal metadata and determine what support for specifying or overriding additional pkg-config dependencies should mean. Done means the requested dependency can be configured through cabal.project.

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