Dynamic linking with a local extra-dep is broken

When using a local package as extra-dep in `stack.yaml` like this:

```yaml
- location: ../../../github/path
extra-dep: true
```

And using the `-dynamic` flag with ghc; stack links with the path library in the snapshot rather than with the local path package specified in stack.yaml, resulting in link errors (e.g. undefined reference) during linking. The linker options passed to gcc use the dynamic library from the snapshot.

Reproduce the case using the local extra-dep in stack.yaml and GHC's -dynamic flag. Inspect how Stack resolves the local package versus the snapshot package and how it constructs the gcc linker options. Done means dynamic linking uses the local extra-dep rather than the snapshot library and no longer produces the reported link errors.

Written by the indexing model from the issue text.