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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