Fix: a timing issue for sequential importLibrary calls

[chrisjshull]

https://github.com/googlemaps/js-api-loader/issues/809

Thank you for opening a Pull Request!

---

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [x] Make sure to open a GitHub issue as a bug/feature request before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [x] Ensure the tests and linter pass
• [x] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)

Fixes #809