Add show dependency tree feature.

[l1xnan]

Add dependency tree feature, be similar to pipdeptree/poetry show --tree/pipenv graph.

$ pipdeptree
Warning!!! Possibly conflicting dependencies found:
* Jinja2==2.11.2
 - MarkupSafe [required: >=0.23, installed: 0.22]
------------------------------------------------------------------------
Flask==0.10.1
  - itsdangerous [required: >=0.21, installed: 0.24]
  - Jinja2 [required: >=2.4, installed: 2.11.2]
    - MarkupSafe [required: >=0.23, installed: 0.22]
  - Werkzeug [required: >=0.7, installed: 0.11.2]
Lookupy==0.1
pipdeptree==2.0.0b1
  - pip [required: >=6.0.0, installed: 20.1.1]
setuptools==47.1.1
wheel==0.34.2

[mitsuhiko]

I do want this. However I think it might have to wait until I have figured out how to rip out pip-tools for something else.