meteor/packages/modules/install-packages.js

function install(name, mainModule) {
  var meteorDir = {};

  // Given a package name <name>, install a stub module in the
  // /node_modules/meteor directory called <name>.js, so that
  // require.resolve("meteor/<name>") will always return
  // /node_modules/meteor/<name>.js instead of something like
  // /node_modules/meteor/<name>/index.js, in the rare but possible event
  // that the package contains a file called index.js (#6590).

  if (mainModule) {
    meteorDir[name + ".js"] = [mainModule, function (require, e, module) {
      module.exports = require(mainModule);
    }];
  } else {
    // back compat with old Meteor packages
    meteorDir[name + ".js"] = function (r, e, module) {
      module.exports = Package[name];
    };
  }

  meteorInstall({
    node_modules: {
      meteor: meteorDir
    }
  });
}

// This file will be modified during computeJsOutputFilesMap to include
// install(<name>) calls for every Meteor package.