Xcode project source file references need to always be relative to the
top of the source tree in order for SCM and debug symbols to work right.
We must even allow the relative paths to cross outside of the top source
or build directories.

For subdirectory project() command Xcode projects we use the source
directory containing the project() command as the top.  Relative paths
are generated accordingly for each subproject.

See issue #8481.