Symbolic version to designate module loaded version

When the Advanced module version specifiers is enabled, the loaded symbolic version may be used to designate the currently loaded version of specified module.

$ ml display foo@loaded
-------------------------------------------------------------------
/path/to/modulefiles/foo/1.0:

module-whatis foo/1.0
-------------------------------------------------------------------

If no version of specified module can be found loaded, an error is returned.

$ ml display foo@loaded
ERROR: No loaded version found for 'foo' module