find_library: Fix repeat call after changing directory content (#15293)
We use cmGlobalGenerator::GetDirectoryContent to avoid repeating directory listings. However, GetDirectoryContent loads content from disk at most once. This breaks find_library calls that occur when disk content has changed since preceding find_library calls. Teach cmGlobalGenerator::GetDirectoryContent to save the directory modification time when content is loaded and re-load content if it changes. Create a RunCMake.find_library test with a case covering this.
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