npm.bbclass: Fix building node modules with npm@5

npm.bbclass: Fix building node modules with npm@5

npm cache clear throws an error with npm@5 and suggests to
use npm cache verify instead. But our cache is actually empty,
so use npm cache clear --force.

npm install in the source directory creates symlinks with npm@5.

Use a combination of npm pack and npm install module-version.tgz
that works the same way with older and new npm versions and is
guaranteed to create actual copies instead of directory symlinks.

This change allows using nodejs 8.x LTS, tested with 8.9.4.

Signed-off-by: Zoltán Böszörményi <zboszor@pr.hu>
Signed-off-by: Ross Burton <ross.burton@intel.com>