gopls/internal/regtest: port @refs to new marker tests

This change adds support for @refs to the new marker test
framework, migrates the old test cases, and adds some new
ones. In many cases the old tests were poorly scoped and
entangled, and this change teases them apart and documents
their purpose.

Also:
- Support variadic marker implementation functions.
- Delete old runner.Reference impl.
- Remove stale references to "v2" of references and implementations.

Change-Id: Ibe30f1be4c9834df3a2e69b983ea3d4bbbcdfb72
Reviewed-on: https://go-review.googlesource.com/c/tools/+/475924
Run-TryBot: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/gopls/internal/lsp/source/methodsets/methodsets.go b/gopls/internal/lsp/source/methodsets/methodsets.go
index af836a5..f8963b9 100644
--- a/gopls/internal/lsp/source/methodsets/methodsets.go
+++ b/gopls/internal/lsp/source/methodsets/methodsets.go
@@ -9,7 +9,7 @@
 // This package provides only the "global" (all workspace) search; the
 // "local" search within a given package uses a different
 // implementation based on type-checker data structures for a single
-// package plus variants; see ../implementation2.go.
+// package plus variants; see ../implementation.go.
 // The local algorithm is more precise as it tests function-local types too.
 //
 // A global index of function-local types is challenging since they