incorrectly pointing +alias doesn't lead to any errors now, which is wrong

[yegor256]

Consider this file:

+alias foo
+package ttt

[] > bar
  foo > @

Here, it is expected that the ttt/foo.xmir file exists. If it doesn't, we will have a runtime error when trying to access foo. This can easily be prevented in compile time. Let's just check that all files referred by +alias metas do exist.