Adapt to the new preparer protocol.

[smashwilson]

As coordinated in deconst/deconst-docs#234, while preserving backwards compatibility.

Essentially, this will change the behavior of the preparer when no content service API key or URL are provided: it will still write metadata envelopes to disk, but:

• It will write them to an ENVELOPE_DIR rather than _build.
• Similarly, it'll copy each asset to an ASSET_DIR.
• It'll inject asset placeholders and asset offets within the serialized envelopes.