#1761: Put `/node_modules` in package root on build

Package code pulled from a git repo does not contain the directory `/node_modules`, as these modules are downloaded during the build process. However, this can confuse NPM prior to build completion.

If the package directory doesn’t  have a '/node_modules' subdirectory, NPM will assume we are not at the package root and will search the directory's parents for one. It will then set the root to any ancestor that does have a '/node_modules' subdir. This can cause the build to fail for downloaded user packages and also Meteor itself if run from checkout.

See also #927