62871df0e6
pyyaml is already vendored but as a consequence we don't benefit from the faster native loader. Introduce the possibility to provide a vendored package as fallback to an optional pypi package. If the pypi package install fails, we can still use the vendored one without the speed benefit. This makes my `mach build export` step 8% faster. Differential Revision: https://phabricator.services.mozilla.com/D234652