It is not possible to use global build-snaps

[Leo Arias]

We can define build-packages globally in the snapcraft.yaml. This means that the build-packages are not for a part in particular, but for all of them.

With build-snaps we can't define them globally. We can only define them inside a part.

To reproduce, try to build the following yaml:

name: build-package
version: '0.1'
summary: install a build-package
description: Install a build-package.

build-snaps: ['hello']

parts:
  hello:
    plugin: nil

[Kyle Fazzari (kyrofa)]

Agreed. This needs to change in the schema to allow for it in the first place, but then the grammar already exists to handle it. We just need to add it to the `GlobalGrammarProcessor`.