fix(kit): resolve relative module paths when installing

[danielroe]

🔗 Linked issue

partly resolves #20883

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Regression introduced in #20757. We need to normalise module relative paths.

There's still an issue where loading an implicit index (e.g. ~/modules/test for ~/modules/test/index) fails owing to jiti trying to read the directory, but I think that's consistent with the ESM ecosystem. (cc: @pi0)

Details

 ERROR  Error while requiring module ./modules/test/: Error: EISDIR: illegal operation on a directory, read                                                             22:35:47


 ERROR  Cannot restart nuxt:  EISDIR: illegal operation on a directory, read                                                                                            22:35:47

  at Object.readSync (node:fs:757:3)
  at tryReadSync (node:fs:438:20)
  at readFileSync (node:fs:484:19)
  at jiti (node_modules/.pnpm/jiti@1.18.2/node_modules/jiti/dist/jiti.js:1:244019)
  at requireModule (packages/kit/src/internal/cjs.ts:108:26)
  at normalizeModule (packages/kit/src/module/install.ts:63:125)
  at async installModule (packages/kit/src/module/install.ts:13:41)
  at async initNuxt (packages/nuxt/src/core/nuxt.ts:339:7)
  at async load (packages/nuxi/src/commands/dev.ts:132:9)
  at async _applyPromised (node_modules/.pnpm/perfect-debounce@1.0.0/node_modules/perfect-debounce/dist/index.cjs:56:10)

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.