Introduce @package header support

[omry]

Package Header

Hydra 1.0 introduces a package header described in details in this design doc.

The header contains the @package directive, defined as:

@package: _global_ | package-path

• _global_: Global package, this is the default behavior in Hydra 0.11
• package-path: Explicit package path, such as oompa.loompa,
  the following keywords are replaced at runtime:
  • _group_: config group in dot notation: foo/bar/zoo.yaml -> foo.bar
  • _name_: config name: foo/bar/zoo.yaml -> zoo

Header format

The header format is generic:

# @oompa a.b.c
# @loompa: yup
x: 10

The resulting header is

{"oompa": "a.b.c", "loompa": "yup"}

Both colon and whitespace are accepted as a separator between the key and the value.
Unrecognized header keys (like oompa and loompa) are ignored.