fix: webpack / rspack load virtual module with '/' prefix on windows platform

[tmg0]

Resolves: #421

• Replace all \ with / after normalizeAbsolutePath function when the virtual module id has a prefix with / in webpack / rspack load function
• Add resolvedId prefix in test/fixtures/virtual-module/unplugin.js

Changes:

id = normalizeAbsolutePath(id)
  if (id.startsWith('\\'))
    id = id.replace(/\\/g, '/')

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/unjs/unplugin@422

commit: 077d35c