have openAndParse use return values rather than channels
have openAndParse use return values rather than channels

Bug: 65683273
Test: build/soong/scripts/diff_build_graphs.sh \
      'build/blueprint:work^^^' 'build/blueprint:work'

Change-Id: I941dfa76b94178198994a280eb40bcded891eda1

danw/blueprint,danw/blueprint,google/blueprint,google/blueprint