feat(codegen): render the module entrypoint

[TomChv]

Wire the entrypoint renderer, ported with the client codegen but never
reachable, to a subcommand. Unlike the binding generators it never sees the
schema: it works from the typedef JSON the SDK introspector emits by scanning
the user's own source, which is what carries the per-declaration locations the
dispatcher imports classes from.

The golden is the introspector's real output, not hand-written JSON — captured
by scanning a module that exercises what dispatch has to handle: a constructor
with a defaulted argument, exposed fields (including an object one, which
round-trips through an ID), an optional argument, an async method and a void
return. Upstream has no test for this renderer at all, here or in
dagger/dagger, so this is the first thing pinning it.

Producing that fixture surfaced two constraints on how the compiler reaches the
scanner, both now recorded in the design: it must be the full package, since a
trimmed one loses lib/lib.*.d.ts and with it every global type (any module
returning Promise fails, confusingly, as "could not resolve type reference
for string"); and it must sit next to introspector.js, since bare imports
resolve from the importing file rather than the working directory.

Signed-off-by: Tom Chauveau tom@dagger.io

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