Add explicit ServiceContextModule dependency to fix Xcode linker error

[William-Laverty]

Summary

Add swift-service-context as an explicit package dependency and ServiceContextModule as a target dependency of AsyncHTTPClient.

Problem

AsyncHTTPClient directly references ServiceContext.current (from swift-service-context) in HTTPClientRequest+Prepared.swift, and also via the default argument of Tracer.withSpan() calls — Swift compiles default arguments into the caller, so ServiceContext.current gets compiled into AsyncHTTPClient.o, creating a direct symbol reference to ServiceContextModule.

However, the package only declares a dependency on Tracing (from swift-distributed-tracing), not on ServiceContextModule directly.

When Xcode builds SPM packages as frameworks, transitive dependencies are not automatically propagated to the linker, causing:

Undefined symbols for architecture arm64:
  "static ServiceContextModule.ServiceContext.current.getter", referenced from:
      implicit closure #1 () -> ServiceContextModule.ServiceContext in default argument 1 of
      (extension in Tracing):Tracing.Tracer.withSpan<A>(...) in AsyncHTTPClient.o

Fix

A two-line change to Package.swift:

1. Add swift-service-context as a package dependency
2. Add ServiceContextModule as a target dependency of AsyncHTTPClient

This is the same class of bug as #721 (missing NIOTLS).

Fixes #890.

[fabianfett]

I think this is a dupe of #891.

[fabianfett]

As stated before this is a dupe of #891.