x/tools/gopls: include modifiers in completion candidate labels

[muirdm]

Consider whether gopls should include full expressions in candidate labels. For example:

var foo func() int
var bar *int
var _ int = <>

At <> we currently offer candidates with labels foo and bar (as of https://go-review.googlesource.com/c/tools/+/323449), but they are inserted as foo() and *bar. We could change their labels to foo() and *bar, respectively.

There will be lots of test fallout from this change. We should consider changing the mark based completion tests to depend less directly on the candidate labels.

/cc @findleyr

[danishprakash]

I'd like to take this up @muirdm

[muirdm]

Go for it. The main challenges are dealing with the tests, and making sure the labels are correct when snippets are disabled.