heisen

fix(toml): remove `lib.plugin` key support and make it warning

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### What does this PR try to resolve? Remove `lib.plugin` key, making it an "unused key" warning. Remove some of the tests, which should look useless (I hope I'm understanding...

fix(toml): Convert warnings that `licence` and `readme` files do not exist into errors

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### What does this PR try to resolve? In this PR: - Changed the warning to a hard error and modified the associated test function; - Removed what should have...

fix(env): cargo:rerun-if-env-changed doesn't work with env configuration

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### What does this PR try to resolve? Fixes https://github.com/rust-lang/cargo/issues/10358 Fixes #8693 ### Additional information Due to my stupidity, I screwed up the [previous PR](https://github.com/rust-lang/cargo/pull/14027). I didn't know what to...