Without this PR, if you have a list of package requirements like:

packages:
  foo:
    require:
    - x
    - y
    ...

x and y must be one_of or any_of like one_of: [+shared], even if there is just a single item in the one_of list. This PR allows specifying strings as the elements of the require: list like:

packages:
  foo:
    require:
    - @version+shared

This is useful if you are combining packages config from multiple files (if there is only one configuration file, then generally you might as well use require: @version+shared rather than specifying the requirements as a list