refactor(errorsx): prepare for splitting the package (#476)

We will move the sane part of this package to i/netxlite/errorsx
and we will move the rest to i/e/legacy/errorsx.

What is the sane part? The sane part is error classifiers plus
the definition of ErrWrapper. The rest, including the rules
on how to decide whether an operation is major, are tricky and
we should consider them legacy and replace them with rules
that are more easy to understand and reason on.

Part of https://github.com/ooni/probe/issues/1591

internal/errorsx/errno_windows.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-07 14:56:43.179707 +0200 CEST m=+0.114644626
+// Generated: 2021-09-07 15:15:03.324258 +0200 CEST m=+0.109328501
 
 package errorsx