feat(ssr): avoid adding unused imports @W-17263761

[wjhsf]

Details

In #4873 we added a feature to remove unused imports, but we avoid unnecessary work if we just don't add them, in the first place.

I also noticed that all we use cxt.hoist for is imports, so I smushed it into cxt.import, instead.

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.
• 💔 Yes, it does introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.
• 🔬 Yes, it does include an observable change.

GUS work item

[nolanlawson]

I was concerned that without optimize-imports.ts we'd have to do a lot of manual bookkeeping, but this looks like a nice improvement. Looks good!