Remove extra log from assets test
Move PACKAGE/.npm/ to PACKAGE/.npm/package/

This is to isolate the NPM dependencies of the package at runtime from the NPM
dependencies of any build-time plugins in the package, which live in
PACKAGE/.npm/plugin/foo.

Before, plugins could see the node modules at PACKAGE/.npm/node_modules, which
was not isolated enough.

(One issue would be that if you happened to switch a package from having runtime
dependencies to having buildtime dependencies, the buildtime dependencies would
not get installed. This is partially an issue because we don't know to uninstall
all dependencies if they are all removed, but even if that is fixed it would
enforce an unnecessary ordering semantics on NPM updates.)