Add the allow_exact_repetitions option to the module_name_repetititions lint

[SLUCHABLUB]

changelog: [module_name_repetitions]: new configuration option allow_exact_repetitions, enabled by default

Closes #14226

[rustbot]

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[samueltardieu]

changelog: new configuration entry: [allow_exact_repetitions]

The changelog should contain the affected lint name, such as in

changelog: [`module_name_repetitions`]: new configuration option `allow_exact_repetitions`, enabled by default

[llogiq]

Apart from the merge conflicts, this looks good to me. Please rebase and ping me.

[SLUCHABLUB]

@llogiq, I have now rebased.

[llogiq]

Thank you!