The type interface between audits/gatherers and computed artifacts is unsound: both audits and gatherers use the full LH.Artifacts type, even though both are actually limited by the runner. At some point I wanted to fix this, but for some unwritten reason the change was not merged.

Recently in #16396 I struggled to add SourceMaps to a computed artifact used by many audits over multiple layers of our modules. Errors looked like "can't call find on undefined", and was many files removed from the audit file that was missing a declaration for "SourceMaps". What helped me make the change was tweaking how computed artifacts work:

1. In makeComputedArtifact, disallow any undefined value given as properties of the dependencies object
2. Change existing undefined / optional properties to instead be nullable

This allows the error to be generated right next to the improper usage. It also forces all callers, including tests, to consider every portion of the dependencies object - this is a little cumbersome but I think is clearly a benefit in that it makes for more accurate tests.

I decided to only throw an error in makeComputedArtifact if isUnderTest is true - in production this will simply log an error message. This is to allow any thing this PR may have missed, but was still otherwise OK, from producing false errors.