AbstractResolver does not enforce build_sdist

[smoparth]

Description
build_sdist is not defined on AbstractResolver. HookSDistResolver and HookPrebuiltResolver don't define it either. Accessing resolver.build_sdist on these classes raises AttributeError. Any future subclass that forgets to add it will have the same problem.

Problem
When downstream code accesses resolver.build_sdist on a hook resolver, it will raise AttributeError.

Fix

1. For HookPrebuiltResolver: add build_sdist = None. Pre-built wheels don't need an sdist build step. This is a fixed value, not user-configurable.
2. HookSDistResolver: add build_sdist = BuildSDist.pep517 as a user-configurable instance field with a default. The hook can return an sdist, a tarball, or a git checkout, the user who writes the hook knows what it returns, so they should specify the build method in YAML.

Example:

source:
  provider: hook-sdist
  build_sdist: tarball    # or pep517 (default)