encoding/protobuf: enforce loading builting packages
encoding/protobuf: enforce loading builting packages

For now it loads all. This avoids renumbering (confusing
while debugging) and potential other misses.

Fixes #635

Change-Id: I8dd67f023a2c3667e9af4f530cd51c6f28206f2b
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8232
Reviewed-by: CUE cueckoo <0f32a827c9a5f00281f936ac78b5ef3ba80a954c@gmail.com>
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue