Expose Marker tree structure in the API

I'd like to be able to use a library like packaging in Debian's dh-python, to parse environment markers, rather than having to maintain our own hacky parser.

The use-case is parsing .dist-info metadata from a built package, during the build, to generate Debian package dependencies based on the Python module's declared dependencies.

Similarly to #448, this means we're evaluating the markers for all supported environments, not just the current environment. So instead of a boolean evaluate(), we need to:

1. Determine whether this dependency is required at all (boolean).
2. Determine what python_version constraints are applied to the module, and emit appropriate Debian dependencies, as possible.

Examples:

1. Requires-Dist: foo; (os_name == 'posix') should result in a dependency on python3-foo.
2. Requires-Dist: bar; python_version < '3.5' should result in a dependency on python3-bar | python3 (>= 3.5)

This means evaluate() is insufficient, for our needs. I really need to be able to walk the tree.

Can we make more of the tree structure public API?

Start by reviewing the existing evaluate() API and the marker-tree behavior referenced by issue #448. Determine which tree structure must become public so callers can inspect all supported environments and derive dependency constraints; done means the public API supports traversal beyond a boolean evaluation, with coverage for the requested use cases.