BaseTools:Add [packages] section in dsc file

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=2270

Currently a PCD (e.g. FeaturePCD) cannot be used in a conditional
statement in a DSC/FDF file without a module in the build referencing
the PCD package DEC file.

An example implementation that to support this is to allow a [Packages]
section in the DSC file to list additional package dependencies for PCD
references in the package DSC/FDF files.

this patch is going to  add the ability to have the [packages] section
defined in the DSC file

Cc: Liming Gao <liming.gao@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Acked-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/AutoGen/PlatformAutoGen.py

@@ -975,6 +975,7 @@ class PlatformAutoGen(AutoGen):
                 continue
             ModuleData = self.BuildDatabase[ModuleFile, self.Arch, self.BuildTarget, self.ToolChain]
             RetVal.update(ModuleData.Packages)
+        RetVal.update(self.Platform.Packages)
         return list(RetVal)
 
     @cached_property