Stop scanning node_modules directories in PackageSource#_findSources.

[benjamn]

This functionality was originally intended to allow importing compiled-to-JS modules from node_modules, by precompiling any modules found in top-level npm packages, as long as a Meteor compiler plugin was registered for the module's file extension.

As discussed in #9800, this extra compilation burden can be non-trivial, especially if you happen to install an npm package such as less, which contains hundreds of .less files in the node_modules/less/test/ directory.

More generally, this functionality was an early attempt to enable selective compilation of node_modules directories, but it was not a good solution to that problem, because in almost all cases the extra compilation was unwanted.

Meteor 1.7 (formerly known as 1.6.2) will give full control over selective compilation of node_modules back to the application developer (#9771), which should afford a much better solution to this problem. If you've installed some .less or .scss or .ts files from npm into your node_modules directory, just create a symlink to the package directory within your application, and those modules will be compiled and become importable by other JS modules, as if they were part of the application.