[gopls-release-branch.0.19] gopls/internal/settings: revert to old goimports index

There have been additional nil dereferences in the new logic
(beyond those fixed in CL 680258) and the fix looks nontrivial.

This is a partial revert of CL 678475.

Fixes golang/go#74055

Change-Id: I95906e4302f8225edd6e512104db6808d512c280
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682715
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
(cherry picked from commit b021ce4)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/682755

Author: adonovan

gopls/internal/settings/default.go

@@ -39,7 +39,7 @@ func DefaultOptions(overrides ...func(*Options)) *Options {
 				DynamicWatchedFilesSupported:               true,
 				LineFoldingOnly:                            false,
 				HierarchicalDocumentSymbolSupport:          true,
-				ImportsSource:                              ImportsSourceGopls,
+				ImportsSource:                              ImportsSourceGoimports,
 			},
 			ServerOptions: ServerOptions{
 				SupportedCodeActions: map[file.Kind]map[protocol.CodeActionKind]bool{

gopls/internal/test/marker/testdata/completion/issue62676.txt

@@ -53,7 +53,7 @@ import "os"
 
 func _() {
 	// This uses goimports-based completion; TODO: this should insert snippets.
-	os.Open(${1:}) //@acceptcompletion(re"Open()", "Open", open)
+	os.Open //@acceptcompletion(re"Open()", "Open", open)
 }
 
 func _() {