Add deployable tarballs to pack-next (#93963)
### What?
Adds a `--deployable-tar` option to `pack-next` that writes generated
tarballs into the target project and patches package references to use
relative `file:` paths.
### Why?
Projects that need to deploy with locally packed Next.js packages should
be able to include those tarballs inside the project directory instead
of referencing tarballs from the Next.js checkout.
### How?
Resolves the target project package.json location, writes tarballs to a
sibling `tarballs/` directory, and maps package override paths to
project-relative references when patching package.json. Existing `--tar`
behavior continues to write to the repository tarballs directory.
### Verification
- `git diff --check`
- `git diff --cached --check`
- `pnpm prettier --with-node-modules --ignore-path .prettierignore
--write scripts/pack-next.ts scripts/pack-utils/patch-package-json.ts
contributing/core/testing.md contributing/core/developing.md`
- `npx eslint --config eslint.config.mjs --fix scripts/pack-next.ts
scripts/pack-utils/patch-package-json.ts contributing/core/testing.md
contributing/core/developing.md` (Markdown files were ignored because no
matching ESLint config was supplied)
- `pnpm pack-next --project ../sandbox/nextjs-duplicated-html-repro/
--deployable-tar`
<!-- NEXT_JS_LLM_PR -->
