Temporarily allow modification of a BuildOutput's raw dependencies

[parlough]

The issue in #168 was not caught due to the packages depending on each through through pub rather than as path dependencies. It was the native_toolchain_c tests that failed which earlier on CI depended on a version of native_assets_cli during test.

The core issue here is that Dependencies should likely be properly immutable (as indicated by its const constructor that's always been there) and its dependencies property should instead be exposed as an Iterable or not at all directly.

Since making those changes is a breaking change, I will instead follow-up with those changes as part of a 0.4.0 release. This fix removes the const lists/maps that the depending packages are not currently expecting.

Fixes #168

[parlough]

For extra validation I locally modified native_assets_builder and native_toolchain_c to use a path dependency on the updated native_assets_cli and ran their complete test suites.

As I mentioned in #168 (comment), it might be a good idea to explore how to do this in CI (if/when it makes sense?).

[dcharkes]

Thanks Parker!

[coveralls]

coverage: 98.079%. first build when pulling ab90f30 on parlough:fix/168 into 762b4da on dart-lang:main.