This change completely breaks hashstack... Here is the breakage:
```
ondrej@kittiwake:~/repos/hashstack(master)$ hit build
[ERROR] Uncaught exception:
Traceback (most recent call last):
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/cli/main.py", line 169, in help_on_exceptions
    return func(*args, **kw)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/cli/main.py", line 143, in main
    retcode = args.subcommand_handler(ctx, args)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/cli/frontend_cli.py", line 40, in run
    self = cls(ctx, args)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/cli/frontend_cli.py", line 36, in __init__
    self.builder = ProfileBuilder(self.ctx.logger, self.source_cache, self.build_store, self.profile)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 27, in __init__
    self._load_packages()
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 49, in _load_packages
    visit(pkgname)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 45, in visit
    visit(dep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 45, in visit
    visit(dep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 45, in visit
    visit(dep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/builder.py", line 42, in visit
    spec = package.PackageSpec.load(self.profile, use)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/package.py", line 42, in load
    doc, hook_files = load_and_inherit_package(profile, name, package_parameters)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/package.py", line 176, in load_and_inherit_package
    doc = profile.load_package_yaml(package_name, parameters)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/spec/profile.py", line 61, in load_package_yaml
    load_yaml_from_file(filename, parameters) for filename in result.values()]
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/formats/marked_yaml.py", line 195, in load_yaml_from_file
    return marked_yaml_load(expanded_stream, filecaption)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/formats/marked_yaml.py", line 186, in marked_yaml_load
    return MarkedLoader(stream, filecaption).get_single_data()
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 39, in get_single_data
    return self.construct_document(node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 43, in construct_document
    data = self.construct_object(node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 88, in construct_object
    data = constructor(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/formats/marked_yaml.py", line 136, in construct_yaml_map
    obj, = SafeConstructor.construct_yaml_map(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 401, in construct_yaml_map
    value = self.construct_mapping(node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 211, in construct_mapping
    return BaseConstructor.construct_mapping(self, node, deep=deep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 133, in construct_mapping
    value = self.construct_object(value_node, deep=deep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 88, in construct_object
    data = constructor(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/formats/marked_yaml.py", line 140, in construct_yaml_seq
    obj, = SafeConstructor.construct_yaml_seq(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 396, in construct_yaml_seq
    data.extend(self.construct_sequence(node))
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 118, in construct_sequence
    for child in node.value]
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 88, in construct_object
    data = constructor(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/formats/marked_yaml.py", line 136, in construct_yaml_map
    obj, = SafeConstructor.construct_yaml_map(self, node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 401, in construct_yaml_map
    value = self.construct_mapping(node)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 211, in construct_mapping
    return BaseConstructor.construct_mapping(self, node, deep=deep)
  File "/auto/nest/nest/u/ondrej/repos/hashdist/hashdist/deps/yaml/constructor.py", line 136, in construct_mapping
    "found already in-use key (%s)" % key, key_node.start_mark)
ConstructorError: while constructing a mapping
  in "/auto/nest/nest/u/ondrej/repos/hashstack/pkgs/openssl.yaml", line 11, column 3
found already in-use key (after)
  in "/auto/nest/nest/u/ondrej/repos/hashstack/pkgs/openssl.yaml", line 14, column 3

This exception has not been translated to a human-friendly error message,
please file an issue at https://github.com/hashdist/hashdist/issues pasting
this stack trace.
```