fix: support import maps in npm module resolution

[eduardoboucas]

Which problem is this pull request solving?

This is a follow-up to #454 addressing a couple of issues I found when testing this in production.

The main change is that the esbuild plugin needs to be aware of user-defined import maps, otherwise it will fail to resolve specifiers that have been remapped.

I ended up making other changes, which I'll describe using inline comments.

[eduardoboucas]

This is a bit annoying, but we're using @import-maps/resolve to resolve import maps, and it expects URL objects, whereas getContents() returns URLs as strings.

I suppose I could've changed getContents() to use URL objects, since they should be converted to strings automatically when marshalled to JSON. I felt that creating a separate method would be an easier first take, but I might still do that as a follow-up.

[eduardoboucas]

Moved this up so that we can pass it as outdir to the first build call. That shouldn't be needed, because we're setting write: false, so we won't write any files to disk anyway. But esbuild still complained if I didn't set an outdir, which I think is a bug.

[eduardoboucas]

This will point to a forums post where we explain the feature and the fact that it's experimental.

[Skn0tt]

Looks good! Thanks for all the comments, super helpful in reviewing.