Global source-repository-packages

### Summary

When overriding dependencies using `source-repository-package`, the overridden packages become local packages.

This means that neither the source code or build results get cached between projects.
It also makes it difficult for `nix` tooling (or similar) to provide pre-cached packages as cabal-install will be reluctant to use a version of a package coming from a package database provided by this tooling.

### Proposal

Add a setting to `source-repository-package` stanzas that allow setting it to be local or global.

Default this to local if it depends on a local package, ie, to enforce a local package transitive closure property.

I think this default would work well for I think the two main use cases:
- overriding an external dependency (global)
- bringing in a tightly coupled sibling package (local)