This changes a few things about "components":
* Rename what was previously ExternalResource to CustomResource,
and all of the related fields and parameters that this implies.
This just seems like a much nicer and expected name for what
these represent. I realize I am stealing a name we had thought
about using elsewhere, but this seems like an appropriate use.
* Introduce ComponentResource, to make initializing resources
that merely aggregate other resources easier to do correctly.
* Add a withParent and parentScope concept to Resource, to make
allocating children less error-prone. Now there's no need to
explicitly adopt children as they are allocated; instead, any
children allocated as part of the withParent callback will
auto-parent to the resource provided. This is used by
ComponentResource's initialization function to make initialization
easier, including the distinction between inputs and outputs.
301739c6b5