Put `/node_modules` in package root on build

Fixes #1761.

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.