loader: avoid race condition when loading package list

I was seeing bad error messages... but only some of the time. So I went
digging. Eventually I traced it to this one bug, probably introduced in
https://github.com/tinygo-org/tinygo/pull/5290.

This is not just a visual issue, this is actually a bug in TinyGo that
will result in non-reproducible binaries and random behavior
differences.

loader/goroot.go

@@ -245,7 +245,7 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		"internal/cm/":                false,
 		"internal/futex/":             false,
 		"internal/fuzz/":              false,
-		"internal/itoa":               false,
+		"internal/itoa/":              false,
 		"internal/reflectlite/":       false,
 		"internal/gclayout":           false,
 		"internal/task/":              false,