Prepare .bzl files to the new semantics of += on lists
Prepare .bzl files to the new semantics of += on lists

In the future += on lists will work like .extend (by mutating the list) instead
of copying the list items to a new list (current behavior). Some .bzl files
need to be cleaned up to be compatible with both behaviors.

PiperOrigin-RevId: 168530907

google/jsinterop-generator,google/jsinterop-generator,google/jsinterop-generator