Fix gatsby-plugin-mdx not running plugins on findImports(...)

[lucasterra]

Description

This makes findImports(...) also run the run(...) phase from unified/remark. This is needed to allow plugins to inject ESM imports via AST modifications. I have also added a test to cover this use case.

Documentation

#25437 broke a use case that I had a custom plugin for. Basically, I need to inject imports via a plugin to enable code demos.

[pvdz]

Thank you for the test case. Super helpful 💜

[johno]

Thank you for the fix and including a test! 🙏🙏🙏

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