fix: client side load module race condition

[giulianok]

Description

When externals fallbacks are being loaded, a module load could be triggered which could also load external fallbacks that shouldn't be loaded since they are being currently loaded. This change makes sure that external fallbacks are loaded before starting loading any additional module(s).

Motivation and Context

Avoid race condition that could load duplicate external fallbacks

How Has This Been Tested?

Manual testing and unit test

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (adding or updating documentation)
• Dependency update

Checklist:

• My change requires a change to the documentation and I have updated the documentation accordingly.
• My changes are in sync with the code style of this project.
• There aren't any other open Pull Requests for the same issue/update.
• These changes should be applied to a maintenance branch.
• This change requires cross browser checks.
• This change impacts caching for client browsers.
• This change adds additional environment variable requirements for Holocron users.
• I have added the Apache 2.0 license header to any new files created.

What is the Impact to Developers Using Holocron?

[JAdshead]

Some thoughts on using browserIntegrity:
could updates to the bundler alter the build output resulting in the same package ending with different integrity values? If this is the case would name-version be a more reliable key ?

[giulianok]

yeah could happen, and i think name-version or something similar would make it easier to debug and less prune to mismatches