How do I instruct cabal-install or stack to use a local version of a hackage package?

Say I want to tinker with a few hackage packages that depend on each other and run

stack unpack foo-1.2.0

and

stack unpack bar-1.0.0

where bar depends on foo.

Now, to build my modified bar with my modified foo, I specify in my bar.cabal:

build-depends:

  foo == 1.2.0

and add to my bar's stack.yaml:

packages:
  - .
  - rel/path/to/foo-1.2.0/

This works, and builds fine but it seems that if a dependency of bar, say some-package, itself depends on foo it will default to the hackage version rather than compile with my modified version despite my modified version meeting the version constraints. Is there a way to force other dependencies further up the chain to use my local version of the package during the build process? I appreciate this is a very unusual use case.

Weirdly I'm able to build fine but trying to load into stack ghci fails with:

Couldn't match type `foo-1.2.0:Some.Type.InFoo.aDataType`
               with `aDataType`
NB:    `aDataType` is defined at
       `full/path/to/foo-1.2.0/Some/Type/InFoo.hs:(23,1)-(28-60)`
       `foo-1.2.0:Some.Type.InFoo.aDataType`
         is defined in `Some.Type.InFoo`
            in package `foo-1.2.0`
       Expected type: `aDataType`
       Actual type: `foo-1.2.0:Some.Type.InFoo.aDataType`