rework the platform hack for modern conda

[mcg1969]

We can greatly simplify the approach used to compute conda solves for other platforms. For quite a long time, conda has supported the CONDA_SUBDIR environment variable to specify the platform for which to do a solve.

[mcg1969]

Drat, need to fix the msys2 issues

[AlbertDeFusco]

Ah, ha! I figured out how to get this to work. The CONDA_CHANNELS env var acts like -c on the CLI to provide additional channels. I'll put this in a new PR along with a fix to streaming_popen to support setting env vars without modifying os.environ globally.

> cat ~/.condarc
channels:
  - defaults

> conda search tranquilizer
Loading channels: done
# Name                       Version           Build  Channel             
tranquilizer                   0.5.0            py_0  pkgs/main           
tranquilizer                   0.5.0    pyhd3eb1b0_0  pkgs/main

> conda search -c conda-forge tranquilizer
Loading channels: done
# Name                       Version           Build  Channel             
tranquilizer                   0.3.0            py_0  conda-forge         
tranquilizer                   0.3.1            py_0  conda-forge         
tranquilizer                   0.3.1            py_1  conda-forge         
tranquilizer                   0.3.2            py_0  conda-forge         
tranquilizer                   0.3.3            py_1  conda-forge         
tranquilizer                   0.4.0            py_0  conda-forge         
tranquilizer                   0.4.1            py_0  conda-forge         
tranquilizer                   0.4.2            py_0  conda-forge         
tranquilizer                   0.5.0            py_0  conda-forge         
tranquilizer                   0.5.0            py_0  pkgs/main           
tranquilizer                   0.5.0    pyhd3eb1b0_0  pkgs/main           
tranquilizer                   0.6.0    pyhd8ed1ab_0  conda-forge         
tranquilizer                   0.7.0    pyhd8ed1ab_0  conda-forge 

> CONDA_CHANNELS='defusco' conda search -c conda-forge tranquilizer
Loading channels: done
# Name                       Version           Build  Channel             
tranquilizer                   0.3.0            py_0  conda-forge         
tranquilizer                   0.3.1            py_0  conda-forge         
tranquilizer                   0.3.1            py_1  conda-forge         
tranquilizer                   0.3.2            py_0  conda-forge         
tranquilizer                   0.3.3            py_1  conda-forge         
tranquilizer                   0.4.0            py_0  conda-forge         
tranquilizer                   0.4.1            py_0  conda-forge         
tranquilizer                   0.4.2            py_0  conda-forge         
tranquilizer                   0.5.0            py_0  conda-forge         
tranquilizer                   0.5.0            py_0  pkgs/main           
tranquilizer                   0.5.0    pyhd3eb1b0_0  pkgs/main           
tranquilizer                   0.6.0    pyhd8ed1ab_0  conda-forge         
tranquilizer         0.6.0+10.gc568a61.dirty    pyh39e3cac_0  defusco             
tranquilizer                   0.7.0    pyhd8ed1ab_0  conda-forge

[AlbertDeFusco]

This has been implemented by #352