Does not warn/error when no apps are defined

[Evan]

Consider the following yaml:

name: bcal
base: core18
version: git
summary: Storage and general-purpose calculator
description: |
  `bcal` (*Byte CALculator*) is a REPL CLI utility for storage expressions,
  unit conversions or address calculations. If you can't calculate the hex
  address offset for (512 - 16) MiB, or the value when the 43rd bit of a
  64-bit address is set mentally, `bcal` is for you.

grade: stable
confinement: strict

parts:
  bcal:
    plugin: make
    build-packages: [clang, libreadline-dev]
    source: .

I've forgotten to create some apps, but I won't know this until after build and install. We should:

* Add this check to snapcraft lint
* At your option, warn or error when this is encountered in `snapcraft`