You should consider using $q.all for such cases where you can gather up all the promises in single array of promise and then pass it in $q.all & return resultant promise. It will ensure that you're returning data after all promises are done.

In your code, you're just executing your asynchronous ajax calls without worrying about whether they are completed or not. And right after that you're returning detailedPackages data. But obvious it is gonna be empty.

Code

function getPackageDetails(packages) {
  var detailedPackages = {};
  var promises = []; //promise array
  angular.forEach(packages, function(p) {

    var promise = vsmsPackageFactory.getPackage(p.id)
    .then(function(response) {
      var subsystem = response.data.packageManifest.subsystem;
      var partNumber = response.data.packageManifest.resultConfig.partNumber;
      var packageName = response.data.packageMetaData.packageName;
      if(detailedPackages[subsystem] === undefined) {
        detailedPackages[subsystem] = {};
      }
      if(detailedPackages[subsystem][partNumber] === undefined) {
        detailedPackages[subsystem][partNumber] = {};
      }
      detailedPackages[subsystem][partNumber][packageName] = response.data;
    });
    promises.push(promise);
  });
  return $q.all(promises).then(function(){
    return detailedPackages;  // returned data after all promises are done
  });
}

this could be achive using promise all, here.

 var promises = [];
 promises.push(new Promise(function (resolve, reject) {
   vsmsPackageFactory.getPackage(p.id)
    .then(function(response) {
     ...
     resolve('If you want to return something')
    });
 });
 //After the loop it's finished you execute your promises
 Promise.all(promises).then(function (values) {
  ... perform more code
 });