Since commit v3.6.0-rc1~287^2 (GetPrerequisites: Fix
gp_resolved_file_type on non-canonical paths, 2016-03-08) we
accidentally convert relative paths (e.g. system dll file names) to
absolute paths even when we do not know the base directory.  Fix this by
canonicalizing only paths that are already absolute.

Closes: #16240