[Segment Cache] Omit from bundle if flag disabled
Alternative to https://github.com/vercel/next.js/pull/76597.
This works by importing all Segment Cache related code through a single
entry point (segment-cache.ts), and wrapping each export in a feature
flag check.
This fixes the size regression in v15.2 but long term I don't love this
approach. Since experimental flags are an essential part of our
workflow, we should establish a better pattern for dead code
elimination. Ideally it would be done at the bundler level, like how
React's build process works. In the React repo, you don't even need to
add any extra configuration per experiment — if the code is not
reachable, it gets stripped from the build automatically by Rollup. Or,
shorter term, we could stub out experimental modules at build time by
updating the build config, i.e. a more automated version of what I'm
doing manually for this particular flag.
