Fix more path resolution for included submodules
Now with more comprehensive testing! This adds tests for includes within modules. Previous testing was not comprehensive enough since submodules that use `include!` didn't actually work either! The `ModDir` used for resolving mods relative to included files has to be `ModDir::root()`. The original test just so happened to put the submodules in the root which made this work, but if you put the `include!` inside a `mod` block it didn't work.
With this change, when collecting a macro expansion, if the macro call is an `include!`, we use the `ModDir::root()` instead of the current module we're in.
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