A further problem with dynamically inserted import maps was discovered where
sometimes module scripts would never execute. This happens when the script is
still being fetched when the import map is added.
To fix this, cancel all fetching module preloads as well when an import map is
registered and remove them from the module map. In theory this shouldn't be
necessary but I wasn't able to make the tests pass without this step.
For simplicity also remove all module preloads from the scriptloader's list of
preload requests rather than detecting and ignoring them later on.
Differential Revision: https://phabricator.services.mozilla.com/D204202
2d8c18b5d5