[build] Rename zircon/system/banjo/* directories/targets to match library names

The old - names were only ever used because of limitations of the
old Zircon build system, which is no longer an issue.  The target
name in banjo_library() must always match the library name.  For
public Banjo libraries, the directory name matches that name too.

Change-Id: Ifff61436ec7d15eb53dfd21349c4367047e691d4