Refactor how bundles and frameworks are supported.

Make handling of directory separators consistent between
non-bundle and bundle code.

Remove xcode specific flag from cmTarget when getting install_name.

Add (more) consistent convenience functions in cmTarget to get
directories inside of bundles and frameworks to add files to.

This refactor also fixes bug #12263 where frameworks
had the wrong install name when SKIP_BUILD_RPATH.

Also make install_name for frameworks consistent between Makefile
and Xcode generator.
Merge topic 'framework-refactor' into next

355fca6 Refactor how bundles and frameworks are supported.