write out isopack-buildinfo.json

our replacement for buildinfo.json

it's not right yet: unibuilds don't merge in the plugin watchset.
unibuilds need to be rebuilt if their plugins are

Basically, this branch is moving back to the pre-0.9.0 model where you
tell if a dependent package has changed by explicitly putting its
relevant files in your watchset, instead of by including a "build
identifier" which requires a relatively complex calculation.

Notably, let's say you had package P with a plugin, and a package Q that
depends on it.  If a file in P that's neither part of the plugin nor the
package.js changes, you don't have to rebuild Q... but it did change the
0.9.0 "build identifier", which caused an unnecessary rebuild!
add comment about build identifier
remove some logs
Don't need to build deps of versioned packages