internal/lsp: do not return interface itself when finding implementations

Finding implementations of an interface should not give the interface itself
as an implementation.

Fixes #35601

Change-Id: Id3352619ece90fb7ca906ddabca613742d156c76
Reviewed-on: https://go-review.googlesource.com/c/tools/+/208667
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/implementation.go

@@ -77,6 +77,10 @@ func (i *IdentifierInfo) Implementation(ctx context.Context) ([]protocol.Locatio
 		if err != nil {
 			return nil, err
 		}
+		// Do not add interface itself to the list.
+		if ident.Declaration.spanRange == i.Declaration.spanRange {
+			continue
+		}
 		locations = append(locations, protocol.Location{
 			URI:   protocol.NewURI(ident.Declaration.URI()),
 			Range: decRange,