Make conda tracer compatible with newer conda versions

[kyleam]

We've been pinning the miniconda URL since a6a571709 (2019-08-07) because our tests fails with the newer conda versions. This series should bring the tests into a passing state with the latest conda.

  [1/7] BF: conda: Add extension to temporary environment file
  [2/7] RF: conda: Assume environments are in $root/envs
  [3/7] RF: conda: Fill in editable packages
  [4/7] OPT: conda: Avoid a call to pip
  [5/7] TST: conda: Update some versions used in install test
  [6/7] RF: conda: Avoid calling 'conda install' on root environment
  [7/7] MNT: conda: Unpin conda URL

Fixes #446.

[codecov[bot]]

Codecov Report

Merging #508 into master will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   89.64%   89.62%   -0.02%     
==========================================
  Files         148      148              
  Lines       12209    12216       +7     
==========================================
+ Hits        10945    10949       +4     
- Misses       1264     1267       +3     

Impacted Files	Coverage Δ
reproman/distributions/tests/test_conda.py	100.00% <ø> (ø)
reproman/distributions/conda.py	92.27% <100.00%> (-1.90%)	:arrow_down:
reproman/distributions/piputils.py	100.00% <100.00%> (ø)
reproman/utils.py	86.86% <0.00%> (+0.16%)	:arrow_up:
reproman/resource/ssh.py	89.16% <0.00%> (+0.83%)	:arrow_up:

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