PR Summary

Avoid IEnumerable<string> -> ToArray -> IEnumerable<string> round-trip.

• Replace GetDirectories with EnumerateDirectories
• Replace GetFiles with EnumerateFiles

PR Context

I considered removing the Linq methods from the class, but GetDirectories and GetFiles are implemented using Linq ToArray and an internal method IEnumerable<string> InternalEnumeratePaths, so we have to load the Linq assembly anyway.

Contributes to #14318.

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