first cut at getting cdp-based packages

[doutriaux1]

@zshaheen this is a first cut at what @williams13 wants, a list of all package that use cdp.

We/you can add option to bring all/any of them in.

We should probably have the command output as json (add --json) and then parse this so that we have the channels. I'll let you play with it, but here's the bare-bones:

cdp_manage -l

returns

Fetching package metadata ...............
acme_diags
acme_diags-nox
pcmdi_metrics

And the current help:

cdp_manage -h
usage: cdp_manage [-h] [-l] [-c CHANNEL [CHANNEL ...]]

CDP-based conda packages manager

optional arguments:
  -h, --help            show this help message and exit
  -l, --list            List all cdp-based packages (default: False)
  -c CHANNEL [CHANNEL ...], --channel CHANNEL [CHANNEL ...]
                        Channel where to look (default: ['pcmdi', 'conda-
                        forge', 'acme'])

[doutriaux1]

@zshaheen we need to get this going again.