Remove OpenStruct from Uses cmd

[dduugg]

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Relates to #18777

Replaces the use of OpenStruct in the uses command with a T::Struct. AFAICT, the OpenStruct defined all the necessary methods, so it isn't necessary to introduce add'l functionality.

(Some other minor cleanup is included.)

[dduugg]

Alas, I couldn't figure out how to induce FormulaUnavailableError organically, so I used an expectation here.

Additionally, it doesn't hook when using brew in a test, so I created a command instance and directly invoked run.