Bug 1515: Properly resolving prefix for submodule

Every Yang submodule contains belongs-to statement. Belongs-to statement contains
mandatory prefix statement. Now submodule prefix is resolved properly and set
as module prefix for parsed submodule.

Test models foo and subfoo were modified to cover bug scenario. In model foo there is typedef defined and called
in submodule subfoo.
Ordering test had to be modified accordingly.

Change-Id: I364337c724407ff9ad2c489968e17f1ed060f347
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>