Re: [PATCH v1] crosvm: replace monolithic DEPENDS with PACKAGECONFIG flags

Thanks Keerthivasan — this cleanly addresses the TODO we left in v4 and
it's pretty close for a v1. A few questions before I take it; so let's
do them in a v2.

1. protobuf-native

   The old DEPENDS carried protobuf-native unconditionally, and the
   new base DEPENDS drops it without mapping it into any PACKAGECONFIG
   entry. Please confirm whether it's actually unused (in which case
   removing it as dead weight is fine — a one-liner in the commit
   message noting so would be great), or whether it's needed by a
   specific feature (in which case it should be attached to that
   feature's PACKAGECONFIG line, not left dangling).

   The test you ran ("default PACKAGECONFIG boots on x86-64") wouldn't
   necessarily exercise this — if some non-default feature (GPU,
   something else) pulls in protobuf codegen via a build.rs, that path
   is off in your test. A quick check with `--all-features` (or the
   full upstream default set) would answer it.

2. config-file in the default set

   Upstream crosvm's default features are approximately:

     ["audio", "balloon", "config-file", "document-features",
      "gpu", "libvda-stub", "net", "qcow", "slirp", "usb"]

   Your PACKAGECONFIG default is "audio balloon document-features qcow
   usb libvda-stub net slirp" — i.e. upstream's default minus gpu
   *and* minus config-file. GPU-off is deliberate and explained; the
   config-file omission isn't. Is that intentional (and if so, why)
   or an oversight? I'd lean toward matching upstream on this so our
   "default minus GPU" claim is exact.

3. document-features in the default set

   document-features is an attribute-macro crate whose sole purpose
   is generating documentation from feature declarations — it's a
   build-time doc helper, not a runtime feature. If this recipe isn't
   building or shipping docs by default (and it isn't), enabling it
   by default doesn't serve anyone. Please drop it from the default
   PACKAGECONFIG set. Keep it as a PACKAGECONFIG entry so someone
   who does want the doc build can flip it on.

I'll wait on a v2. Everything else looks good.

Bruce