[feature] Make this work in "dry-run" mode

[smackesey]

Would be great if this tool could leverage the relatively new pip install --dry-run to give a dep tree for a set of packages specified on the command line.

[kemzeb]

With us transitioning away from using pip API, we would need to approach this differently.

Looking at pip's implementation, they use the resolvelib library to perform dependency resolution. Not sure yet how they snatch packages from PyPI (to store it temporarily), but I believe it may performed in their PipProvider class.

There are some other things I believe we should take from their install subcommand:

• --no-install: So that users aren't forced to consider the packages already installed in their environment
• --index-url: So that we are not dependent on just PyPI and can allow users to choose any PEP 503 compliant repository