Issue #3282677 by phenaproxima: In certain situations, Composer may run with a different PHP interpreter than the one running Drupal

Issue #3275324 by phenaproxima: If the PHP binary is not in the PHP interpreter's PATH, Composer invocations fail

Issue #3259228 by phenaproxima, tedbow: Add functional test coverage of Stage::require() to ensure only the correct dependencies are updated

Issue #3230668 by phenaproxima, tedbow: Split out Composer Stager wiring into Package Manager submodule