chore($translatePartialLoader): merge namespaces from different parts

Now $translatePartialLoader can merge namespaces from different parts instead of
overriding them.

So, parts like
foo : {
  bar : "bar"
}
and
foo : {
  buz : "buz"
}
would result in table like
foo : {
  bar : "bar",
  buz : "buz"
}
instead of
foo : {
  buz : "buz"
}