yaml merge tags are not supported

[Tim Süberkrüb]

The current yaml loading implementation in snapcraft doesn't support yaml merge tags.

Example:
parts:
  base: &base
    plugin: nil
    source: .
  my-part:
    plugin: dump
    <<: *base

Snapcraft fails with the following error message:

could not determine a constructor for the tag 'tag:yaml.org,2002:merge'
  in "snap/snapcraft.yaml", line 20, column 5

The problem lies in the yaml dict constructor implementation: https://github.com/snapcore/snapcraft/blob/master/snapcraft/__init__.py#L433

[Tim Süberkrüb (tim-sueberkrueb)]

See https://github.com/snapcore/snapcraft/pull/1384