cmSystemTools: Implement GetRealPath on Windows

Brad Kingrequested to merge
brad.king/cmake:realpath-windows into master

Use cm::PathResolver's RealPath variant to normalize paths, look up their on-disk case, and resolve symlinks, but without resolving subst drives on Windows.

Also drop GetRealPathResolvingWindowsSubst now that GetRealPath works on Windows.

Fixes: #17206 (closed)
Issue: #18033 (closed)

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