Add quick fix for "Import package:x show y"

[DanTup]

Currently if you reference a function from a package that you haven't imported you get a fix to import it. It's be nice if there was an additional fix that imports with a show for just that function (there's already a fix to add new functions to existing imports, so this would make it easier to minimise what you're importing).

[zoechi]

I almost always use show. I think it makes code much easier to understand when it shows where certain identifiers come from. Especially UI code often has a lot of components/widget imports and imports for other stuff.
I'm in favor of making this more convenient.

[FMorschel]

Related to #55842 and #56608.

[FMorschel]

I think it makes code much easier to understand when it shows where certain identifiers come from.

I think then you can have some interest on:

• Add new "Go to Import" option #56584
• Hovers should show what imports are adding that declaration #56935
• Variable hover should show the origin of the given type #56350

[FMorschel]

https://dart-review.googlesource.com/c/sdk/+/396564