snapcraft pull doesn't save the installed build dependencies

[Leo Arias]

Snapcraft should save the name and versions of all the packages installed, so the package can be audited and recreated. Currently, it saves the stage package dependencies, but not the build package dependencies.

To reproduce:

Write a snapcraft.yaml with the following part:

parts:
  my-part:
    plugin: nil
    build-packages: [hello]
    stage-packages: [hello]

hello has one dependency on gcc-6-base.

Run snapcraft pull

Check the file parts/my-part/state/pull:

!PullState
assets:
  build-packages: [hello=2.10-1]
  source-details: null
  stage-packages: [gcc-6-base=6.0.1-0ubuntu1, hello=2.10-1]
project_options: {deb_arch: amd64}
properties:
  plugin: nil
  source: .
  source-branch: ''
  source-commit: ''
  source-depth: 0
  source-subdir: ''
  source-tag: ''
  source-type: ''
  stage-packages: [hello]
schema_properties: []

Note that the gcc-6-base dependency was only added to the stage-packages.