Poetry show: allow displaying source

Issue Kind

Brand new capability

Description

We want to be able to get all of a projects dependencies that use a specific source. For this effect, I propose adding a new option to poetry show: poetry show --source: to include the source of each dependency listed.

Impact

We want our CI process to ensure there are no outdated packages among our internal dependencies.

Workarounds

Our current workaround is to is to manually filter pyproject.toml with tomlq:

tomlq -r '           
  [.tool.poetry.dependencies, (.tool.poetry.group[]?.dependencies // {})]
  | add
  | to_entries[]
  | select(
      (.value | type=="object" and .source=="mysource")
      or
      (.value | type=="array" and any(.[]; type=="object" and .source=="mysource"))
    )
  | .key
' pyproject.toml

And then to filter poetry show -l's output for internal dependencies.

Start by tracing the poetry show command and how dependency source metadata from pyproject.toml is represented. The work is done when an option displays each listed dependency's source, allowing users to identify dependencies from a specific source.