Fix for package.js in .build directories

I had quite some issues of meteorite finding `package.js` directory in `.build` directory of packages. As a consequence symlinks in `packages` directory were pointing to `.build` directories of packages, which broke things. It really depends on the system how files are traversed, so if `.build` is before top-level `package.js` or not.

This simple change makes sure it does not try to search in `.build` directory.