BaseTools: support private package definition

BaseTools: support private package definition

EDKII build spec and DEC spec updated to support private package
definition.
If GUID, Protocol or PPI is listed in a DEC file, where the  Private
modifier is used in the section tag ([Guids.common.Private] for example),
only modules within the package are permitted to use the GUID, Protocol
or PPI. If a module or library instance outside of the package attempts
to use the item, the build must fail with an appropriate error message.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>