Does perform the optional step of fetching descendants and linking:
https://html.spec.whatwg.org/multipage/webappapis.html#fetching-scripts:fetch-the-descendants-of-and-link-a-module-script-2

Partially implements some new destinations ("as" attribute values) for
modulepreload only (not for preload or link attribute reflection)

Differential Revision: https://phabricator.services.mozilla.com/D172368

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1425310
gecko-commit: e1770e1308d6899abd80fb18ca0f5cb4bbd2de93
gecko-reviewers: yulia, smaug, jonco