internal/imports: actually cache resolvers

I forgot to actually save the resolver instances...

Change-Id: Ieff4f22d29038895dc15be97491476f414544a5c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181337
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
diff --git a/internal/imports/fix.go b/internal/imports/fix.go
index 7a465b7..2578166 100644
--- a/internal/imports/fix.go
+++ b/internal/imports/fix.go
@@ -539,14 +539,17 @@
 		return e.resolver
 	}
 	if e.ForceGoPackages {
-		return &goPackagesResolver{env: e}
+		e.resolver = &goPackagesResolver{env: e}
+		return e.resolver
 	}
 
 	out, err := e.invokeGo("env", "GOMOD")
 	if err != nil || len(bytes.TrimSpace(out.Bytes())) == 0 {
-		return &gopathResolver{env: e}
+		e.resolver = &gopathResolver{env: e}
+		return e.resolver
 	}
-	return &moduleResolver{env: e}
+	e.resolver = &moduleResolver{env: e}
+	return e.resolver
 }
 
 func (e *ProcessEnv) newPackagesConfig(mode packages.LoadMode) *packages.Config {