Diff wrapper `getPhetioAPI` assumes a relative path for API file

This issue is to track https://github.com/phetsims/phet-io-wrappers/issues/405 for Natural Selection 1.3.

In https://github.com/phetsims/phet-io-wrappers/issues/405#issuecomment-826008418, @zepumph described that issue as blocking:

I also saw that this is a actually completely different problem from what was originally reported in phetsims/natural-selection#271, and the "APIs are identical" output was actually because of a bug where the "other sim URL" was loading the SAME (this sim's) API file instead of going out and grabbing the other sim's api. I created #406 to solve that, which is also blocking. To fix that, we need this issue to be working though, so both bugs are important improvements, and both blocking NS release.

In 4/23/21 Slack, @zepumph didn't say whether that issue is blocking:

Updates about blocking issues: ...

https://github.com/phetsims/phet-io-wrappers/issues/406 was another diff wrapper bug discovered while working on 405, and I haven't been able to investigate just yet, but looks like a simple-ish bug directly in the diff wrapper.

@zepumph @kathy-phet please clarify: Is this blocking for NS 1.3 ?

phetsims / natural-selection

Diff wrapper `getPhetioAPI` assumes a relative path for API file #274