perf(deps): read each package.json once when resolving dep subpaths

[Cherry]

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• AI was used: Claude Opus 5.5
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Description

With deps.resolveDepSubpath turned on, every subpath import reads and parses the dependency's package.json again just to check for exports. We noticed it in one of our builds, which parsed drizzle-orm's 177 KB package.json over 5,000 times.

This caches that check per package.json for the build, and clears it on buildStart so watch rebuilds still pick up changes. On a fixture with 600 files importing subpaths of one big package, our builds went from ~1.75s to ~0.5s, about the same as with the option off.

Added a test that imports subpaths of a package with exports and one without, from a couple of files, to make sure each package still gets its own answer.

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commit: d9932ed