cmd/go: use local state object in tool.runTool

This commit modifies `tool.runTool` to construct a new modload.State
object using the new constructor instead of the current global
`modload.LoaderState` variable.

This commit is part of the overall effort to eliminate global
modloader state.

[git-generate]
cd src/cmd/go/internal/tool
rf '
  add tool.go:/func runTool\(/-0 var moduleLoaderState *modload.State
  ex {
    import "cmd/go/internal/modload";
    modload.LoaderState -> moduleLoaderState
  }
  add runTool://+0 moduleLoaderState := modload.NewState()
  rm tool.go:/var moduleLoaderState \*modload.State/
'

Change-Id: I0aff1bc2b57d973c258510dc52fb877fa824355c
Reviewed-on: https://go-review.googlesource.com/c/go/+/711128
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

Author: jitsu-net

src/cmd/go/internal/tool/tool.go

@@ -78,9 +78,10 @@ func init() {
 }
 
 func runTool(ctx context.Context, cmd *base.Command, args []string) {
+	moduleLoaderState := modload.NewState()
 	if len(args) == 0 {
 		counter.Inc("go/subcommand:tool")
-		listTools(modload.LoaderState, ctx)
+		listTools(moduleLoaderState, ctx)
 		return
 	}
 	toolName := args[0]
@@ -108,14 +109,14 @@ func runTool(ctx context.Context, cmd *base.Command, args []string) {
 		if tool := loadBuiltinTool(toolName); tool != "" {
 			// Increment a counter for the tool subcommand with the tool name.
 			counter.Inc("go/subcommand:tool-" + toolName)
-			buildAndRunBuiltinTool(modload.LoaderState, ctx, toolName, tool, args[1:])
+			buildAndRunBuiltinTool(moduleLoaderState, ctx, toolName, tool, args[1:])
 			return
 		}
 
 		// Try to build and run mod tool.
-		tool := loadModTool(modload.LoaderState, ctx, toolName)
+		tool := loadModTool(moduleLoaderState, ctx, toolName)
 		if tool != "" {
-			buildAndRunModtool(modload.LoaderState, ctx, toolName, tool, args[1:])
+			buildAndRunModtool(moduleLoaderState, ctx, toolName, tool, args[1:])
 			return
 		}