Allow sourcing from only selected fields in pyproject.toml file

[gbdlin]

Is your feature request related to a problem? Please describe.
Often there are multiple sections inside pyproject.toml file that specify dependencies. Not all of those sections are being used in the project directly, for example dev dependencies group from poetry is mostly populated by tools.

Right now the only way to skip this group is to manually duplicate it into the list of ignored dependencies. Allowing for ignoring this whole section would solve this problem.

Note: it was advised at some point to use a similar solution when having normal and dev dependencies split into separate files, but it's not possible to split the pyproject.toml into multiple files.

The other problem it may fix is a situation when a dependency is defined in the wrong group, for example when a runtime project dependency is defined only in dev section.

Describe the solution you'd like
There should be a way to specify for each pyproject.toml deps source which sections should be allowed or excluded. When using pyproject.toml itself to configure FawltyDeps, it can look like this:

[tool.fawltydeps]
deps = [
    { path = "first/pyproject.toml", exclude = ["tool.poetry.group.dev.dependencies", "tool.poetry.dependencies:some-extra-group"] },
    { path = "second/pyproject.toml", only = ["tool.poetry.dependencies"] },
]

Example above shows how to exclude a specific group in poetry, a specific extras group (that can be used with PEP 621 as well)

Describe alternatives you've considered
The alternative is keeping the ignored list up to date, but that means defining each dev dependency twice. It also doesn't solve the situation when there is some directly used dependency lost in the wrong group, as FawltyDeps won't warn about it if it's not on the ignored list (when for tools, it's obvious you forgot to put one in the ignored lists).

Additional context
None

[jherland]

This is very much in line with #44, except that we'd like to solve this for more than just pyproject.toml dependency groups (and Poetry ones at that). We'd certainly like to provide this, but it's hard to put an ETA on it now.