`getPackageIterator` API forces us to do a refresh, which loads all
packages on the filesystem. However, for `loadGitPackage`'s use case,
we actually can simplify the check quite a bit.

Doing so means that we can now avoid doing a scan for projects
that use SCM-only dependencies.

Test Plan: On master, running the test (without the fix) triggers:
```
core.exception.AssertError@source/dub/test/base.d(790): Trying to access poisoned path: /dub/user/packages/poison/1.0.0/poison/dub.json
```
While with the fix, the test passes.