BaseTools: Move FindExtendTool to GenFdsGlobalVariable.py

BaseTools: Move FindExtendTool to GenFdsGlobalVariable.py

Importing "FindExtendTool" from GenFds.GenFds could create the following
circular imports:

* GenFds.FdfParser => GenFds.Capsule => GenFds.GenFds => GenFds.FdfParser
* GenFds.FdfParser => GenFds.Fd => GenFds.Fv => GenFds.AprioriSection =>
  GenFds.FfsFileStatement => GenFds.GuidSection => GenFds.GenFds =>
  GenFds.FdfParser

This commit moves "FindExtendTool" to GenFdsGlobalVariable.py to break
the circles. Besides, FindExtendTool is tweaked slightly with the
following changes:

ToolDefClassObject.ToolDefDict => ToolDefDict
TAB_GUID => DataType.TAB_GUID
TAB_TOD_DEFINES_TARGET => DataType.TAB_TOD_DEFINES_TARGET
TAB_TOD_DEFINES_TOOL_CHAIN_TAG => DataType.TAB_TOD_DEFINES_TOOL_CHAIN_TAG
TAB_TOD_DEFINES_TARGET_ARCH => DataType.TAB_TOD_DEFINES_TARGET_ARCH

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>