Fix Mac/iOS builds

This change removes the unused "fuchsia_package" GN template. This
change was made to enable the transition of Flutter engine from the Fuchsia
core SDK/IDK to the GN SDK. THe GN SDK contains GN build templates including
a build template also called "fuchsia_package". Because GN template
names are global for a given project this causes a naming collision
that is preventing the GN SDK from being used by Flutter.

The in progress PR to start migrating Flutter engine from the Fuchsia core/IDK
to the GN SDK is here: flutter#19789

Closes flutter#387

Adds `IOStreamedResponse` which is now the type returned by `IOClient`.
It adds a `detachSocket` method forwarding to the inner client.

Fix up duplicate header copy