Fetching necessary dependencies for the whitelist.

[minhoryang]

I'm using conda-mirror for air-gapped computers. (I know it's a special use case rather than just mirroring.)

Config look like:

blacklist:
- name: '*'
platform: osx-64
target_directory: mirrored
upstream_channel: https://repo.continuum.io/pkgs/main/
whitelist:
- build: mkl
  name: _tflow_select
  version: 2.3.0
- build: py36_0
  name: absl-py
  version: 0.7.0
- build: py36_0
  name: astor
  version: 0.7.1
- ...

But after conda-mirroring, I can't install whitelisted packages via mirrored, which weren't covered the dependencies.

[minhoryang]

Here is a snippet for generating a conda-mirror.config.yaml with dependencies