Lint request: Warn when importing inside a macro and not using the $ keyword.

[nilscript]

I suggest adding a lint for when a programmer makes a macro, imports something into said macro and forgets $ before crate keyword. Exporting macros across crates can be a hassel, atleast for me.

[flip1995]

E-hard: It's really hard (maybe impossible) to determine if use statements refer to the current crate or some dependency inside a macro definition. So doing this right is really hard.
E-medium: A basic implementation (nursery/restriction lint), which triggers on every use statement inside a macro is possible. Currently no lint exists, which checks macro definitions, though.

Macro definitions can be checked with EarlyLintPass::check_mac_def.

The challenge here is, that only a TokenStream is available, instead of AST nodes.