Split entrypoint/route handling into separate dev and prod versions (#75169)
Currently, for `handleEntrypoints`, `handlePagesErrorRoute`,
`handleRouteType`, etc, both dev and prod use cases are combined into
the same functions. This leads to a lot of branching, use-case-specific
arguments, etc. for minimal opportunity for shared code.
In a following PR, entrypoint writing for prod will be done through a
single napi call to rust, further branching the two versions.
This splits them formally, at the cost of duplicating a handful of lines
in each case. Perhaps in the future we can develop a better system for
sharing code, but this makes things far clearer and easier to maintain
in this moment.
Test Plan: CI
next.js
f4177614 - Split entrypoint/route handling into separate dev and prod versions (#75169)
