Use `swift package show-dependencies` for searching Commandant
Use `swift package show-dependencies` for searching Commandant

Since the directory to check out Commandant differs depending on the
version of Swift, if you run `swift build` with multiple versions of
Swift without `swift package reset`, there may be multiple checked out
Commandant directories.
That may cause `testCommandantDocsSPM()` to fail.
This change prevents that failure by getting Commandant's directory
used in the test with `swift package show-dependencies`.

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