bindings: small improvements to Python API

[simonas-drauksas-sensmetry]

This PR contains small Python API improvements that I need for integrating Sysand into Syside ReqIF functionality, namely:

1. The ability to obtain the project root (Sysand CLI already can do this through print-root, but not in Python)
2. The ability to obtain a list of dependencies the current project depends on (so I could check if the ReqIF library is imported)

These changes required a bit of changes on the Sysand core / CLI, namely the introduction of "source modes":

• own, which represents the "I only want to list the included files" and was achievable through CLI as sysand sources --no-deps (still possible)
• own_with_deps, which is what the default is sysand sources
• deps, which is new and returns only the dependencies without included files

While the CLI only got a new flag --only-deps and thus there is no breaking change on the CLI side, the Python API lost the include_deps argument and gained no_deps and only_deps instead to better align with the CLI. Not sure if we should consider this a breaking change, or should I try to keep what was already in the Python API?

For context, this is how I would need to get only the deps with the current API:

all_srcs = sysand.sources(root, env_path=root / ".sysand")
own_srcs = sysand.sources(root, env_path=root / ".sysand", include_deps=False)
dep_srcs = [p for p in all_srcs if p not in set(own_srcs)]

While after this PR I will be able to do:

dep_srcs = sysand.sources(root, env_path=root / ".sysand", only_deps=True)

P.S.: The commits are pretty atomic and contain nice commit messages. If not a lot of changes after review are required, I'd like NOT to squash the commits on merge.

[andrius-puksta-sensmetry]

Not sure if we should consider this a breaking change, or should I try to keep what was already in the Python API?

For now I don't consider bindings/core API a part of stability guarantee; too few users to matter.

[andrius-puksta-sensmetry]

gained no_deps and only_deps instead to better align with the CLI

It would be better to use an enum in Python API, otherwise we have to check if both no_deps and only_deps are given. IMO CLI should also change, but since this is breaking, CLI change won't happen until 0.2.0.

[andrius-puksta-sensmetry]

Implementation is fine, but I really don't like the design and defaults. I'd like to make invalid states unrepresentable, my previous comment details what that would look like.

Bindings can be more or less freely altered (very few users), so I would like bindings to have my proposed design, while CLI will wait until 0.2.0 to match. Alternatively, we could release this now, but I would still like to make the change on the next breaking release, and breaking the functionality soon after releasing it is not appealing.