refactor: make `_make_when_spec()` private to `directives.py`

`make_when_spec()` was being used in the solver, but it has semantics that are specific
to parsing when specs from `package.py`. In particular, it returns `None` when the
`when` spec is `False`, and directives are responsible for ignoring that case and not
adding requirements, deps, etc. when there's an actual `False` passed in from
`package.py`.

In `asp.py`, we know that there won't ever be a raw boolean when spec or constraint, so
we know we can parse them without any of the special boolean handling. However, we
should report where in the file the error happened on error, so this adds some parsing
logic to extract the `mark` from YAML and alert the user where the bad parse is.

- [x] refactor `config.py` so that basic `spack_yaml` mark info is in its own method
- [x] refactor `asp.py` so that it uses the smarter YAML parsing routine
- [x] refactor `asp.py` so that YAML input validation for requirements is done up front

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