Fix `module_resolution: "nodenext"` with `mjs` or `cjs` (#71635)
A regression from https://github.com/vercel/turborepo/pull/8748
Closes PACK-3312
Previously, it treated imports to `something.mjs` as importing
`something.mts` with a fallback to `something.js`.
But it should obviously instead fallback to the original extension
`mjs`, which is also what tsc does:
```
Found 'package.json' at 'input/package.json'.
======== Resolving module './src/foo.mjs' from 'input/index.js'. ========
Explicitly specified module resolution kind: 'NodeNext'.
Resolving in CJS mode with conditions 'require', 'types', 'node'.
Loading module as file / folder, candidate module location 'input/src/foo.mjs', target file types: TypeScript, JavaScript, Declaration.
File name 'input/src/foo.mjs' has a '.mjs' extension - stripping it.
File 'input/src/foo.mts' does not exist.
File 'input/src/foo.d.mts' does not exist.
File 'input/src/foo.mjs' does not exist.
File 'input/src/foo.mjs.ts' does not exist.
File 'input/src/foo.mjs.tsx' does not exist.
File 'input/src/foo.mjs.d.ts' does not exist.
File 'input/src/foo.mjs.js' does not exist.
File 'input/src/foo.mjs.jsx' does not exist.
Directory 'input/src/foo.mjs' does not exist, skipping all lookups in it.
======== Module name './src/foo.mjs' was not resolved. ========
```