Clearer identification of missing dependencies (#1588)

Clearer identification of missing dependencies (#1588)

When a complete solution is printed (`alr show|with --solve`), the "external"
section is now named "missing" (which is what it actually meant).

When a diff is show that contains missing dependencies, a new section is
appended containing separately all missing dependencies, labeled as such:
```
$ alr -f with hello unobtanium
Requested changes:

   ✓ hello      ^1.0.2 (add)
   ✓ unobtanium *      (add)

Changes to dependency solution:

   New solution is incomplete.
   +        hello      1.0.2 (new)
   Missing:
   +❗      unobtanium *     (new,missing:unknown)
```