x/tools/gopls: add "Eliminate dot import" code action

[adonovan]

Sometimes we regret using dot imports but they are too tedious to remove. It would be easy (and infallible actually fallible) to offer a code action to automate this transformation. We could also offer the reverse ("Convert to dot import"), but perhaps we should not place temptation in front of weak mortals. (The reverse operation is fallible.)

[findleyr]

Sure, this could be useful, and would be easy to implement as it's just a sequence of simple scope checks and string replacements. I'll note that it wouldn't be (naively) infallible due to shadowing.

In short:

1. search through types.Info.Uses for identifiers from the imported package in question
2. find the scope at that identifier, and check that it does not contain the package name. If it does, either fail the operation or rename the package and start over
3. produce an edit that qualifies the identifier
4. if all qualification was successful, prepend an edit that removes the "." from the import.

As much as I'd like to get nerd-sniped, I'll leave this as a good help-wanted issue. If nobody picks it up, one of us can do it.

[findleyr]

The opposite operation is essentially the same, except that the scope lookup is different, and we must fail if the scope lookup fails. However, since introducing dot imports violates the style guide, I'm less inclined to do it. If someone wants to contribute that direction as well, I'd vote for accepting the feature. (For symmetry, and because I personally am not that opposed to dot imports in tests :)).