Fix rust-analyzer for core libraries

[Danielkonge]

For a description of the problem see: #3985

This fixes the dependency problem for the core libraries for me, but I am not sure if this is the best solution. It might be better to add proper BUILD files describing the core dependencies instead.

(As an aside: It might be nice with some clippy support in rust-analyzer too now that I am adding cargo anyways.)

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[Danielkonge]

I signed the CLA now.