Compiler: hardcode 'da' and 'import_da' as known names.

* da/api.py: break circle import dependency: do no import '.compiler'
  globally, import inside 'import_da' where it is used.

* da/compiler/dast.py(Program): auto add name 'da' to the program scope.

* da/compiler/parser.py: import '..api' to populate
  'common.api_registry'; add 'import_da' as a special api method.

* da/compiler/pygen.py(PythonGenerator.visit_ApiCallExpr): use 'da.'
  prefix for apis instead of 'da.api.'.