Make using python 2 an error

Except if BUILD_BROKEN_USES_SOONG_PYTHON2_MODULES is set, and except for
some core py2 modules that can't be removed until python2 is fully gone.

Bug: 203436762
Test: m nothing
Change-Id: I62ccb6f5687eab1e79c372ffc234a90ca5b566ac
diff --git a/android/variable.go b/android/variable.go
index 1b5d558..8c5c0bc 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -442,6 +442,7 @@
 	BuildBrokenDepfile                 *bool    `json:",omitempty"`
 	BuildBrokenEnforceSyspropOwner     bool     `json:",omitempty"`
 	BuildBrokenTrebleSyspropNeverallow bool     `json:",omitempty"`
+	BuildBrokenUsesSoongPython2Modules bool     `json:",omitempty"`
 	BuildBrokenVendorPropertyNamespace bool     `json:",omitempty"`
 	BuildBrokenInputDirModules         []string `json:",omitempty"`