Warn on conflicting keys of `src/cargo/util/toml/mod.rs`

[0xPoe]

Problem

a few of fields in src/cargo/util/toml/mod.rs have alternative name. When they are set at the same time, we should
warn about conflicting keys.

Possible Solution(s)

Warn when two keys are set that control the same thing.

Notes

From: #10316 (comment)

[0xPoe]

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