This past reliance doesn't work well with Swift, and the sources
were trying to over hand optimize to minimize rebuilds, and that
likely isn't worth the trouble, so explicit imports.
The remaining places that still use them are where they are
needed within the header to deal with relationships between
the local definitions or where there is a cycle between the
headers and it allows either one to be imported first and still
get a complete definition in the using context.
PiperOrigin-RevId: 671379912
`GPBUnknownFields` will be the eventually replacement for `GPBUnknownFieldSet`. This
introduces the type and the changes to `GPBUnknownField`.
The new api will preserve the wire ordering of unknown fields. This is now checked
in conformance tests.
While this adds the type changes and tests them, it does not yet wire the changes
in to the rest of the Runtime, so the conformance tests still done pass.
`GPBUnknownFieldSet` also hasn't been deprecated yet, that will come in later with
the wiring in to the runtime.
PiperOrigin-RevId: 648361455
Thomas Van Lenten