Fix @forward statement altering the scope of the forwarded module

[kketch]

When using the @forward statement multiple times on the same module, there was an unintended mutation of the loaded module scope. Using @forward "path" as prefix-*; would change the scope for a module loaded without a prefix too or with a different prefix, resulting in errors such as "Undefined mixin" or "Undefined function".

I initially found the issue while trying to compile Carbon Design System SCSS style with grass.

The scope built for a particular forward statement in now directly included on the ForwardedModule type just like how its done for the ShadowedModule type.

[connorskees]

Thank you for this change. I will try to release this to crates.io in a bugfix version sometime next week. In the meantime, you should be able to use a git dependency to access these changes.

The test and CLI fixes are appreciated!