Ensure `--deps=none` option works for `bst checkout`

Currently, `bst checkout --deps none` command always produces empty
output. Fix this issue and add regression test for the same.

- element_enums.py: Add Scope.NONE.

- element.py: Ensure Scope.NONE works correctly in addition to
  Scope.RUN/Scope.ALL in Element.dependencies() and Element.search().

- tests/frontend/buildcheckout.py: Fix tests for `--deps none`.

Fixes #670.