Lint: Check that every crate contains at most one export_lint_pass! call

[xFrednet]

The export_lint_pass macro is required to load a lint crate by marker. The macro generates some functions which are marked as #[no_mangle]. If the macro would be called multiple times per crate, it would generate multiple functions with the name. This is unsound (I actually don't know what exactly happens).

A lint should check that each crate contains at most one call to export_lint_pass. It would be cool if we could check that each lint crate defines it exactly once, but I believe that we currently don't have a mechanism to determine if the compiled crate is a lint crate or just a library. Checking at most once, will be sufficient for now :)

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Marker currently has no way to determine, which macro was written in the code. The lint should instead check that each crate contains at most one function called marker_api_version and one called marker_lint_crate_bindings