Add an initial implementation of PEP 735 Dependency Groups

[sirosen]

resolves #23

This initial implementation takes the form of

• treating the dependency-groups table as a Map<String, Vec<DependencyGroupSpecifier>>
• defining DependencyGroupSpecifier as a string or an include

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There are a couple of points I'm not certain about. Although I would be amenable to changing these, I might have to defer to someone more expert, as my Rust is pretty weak.

I don't know if it's satisfactory for the table to be defined as a type alias? It seemed like the best fit, but I'm not sure if there's a better way of doing this.

This is within the SHOULD/MAY bounds of the spec, in that tools are allowed to load and validate all of the data eagerly, but the spec also explicitly prefers lazy validation of the table contents.
There are a few ways I thought it might be possible to tackle this (e.g., add a branch to the DependencyGroupSpecifier enum for a Map<String, toml::Value>), but ultimately it feels like a good fit for Python and a poor fit for Rust.
Eagerly validating the content just seems to me like the most language-appropriate behavior.

[konstin]

Thank you! Looks good, i chose to switch the dependency groups from a type alias to newtype struct pattern, this allows us to add methods such as getting a dep group with resolved includes on it in the future.