Support for external packages and builders

[gaborbernat]

Resolves #2204

• Install package support
• Introduce children package envs
• Add sdist + wheel metadata extractor

[codecov]

Codecov Report

Merging #2235 (243f238) into rewrite (d448d82) will decrease coverage by 0.00%.
The diff coverage is 99.80%.

@@             Coverage Diff             @@
##           rewrite    #2235      +/-   ##
===========================================
- Coverage    99.81%   99.80%   -0.01%     
===========================================
  Files          164      169       +5     
  Lines         9553     9791     +238     
  Branches       980     1001      +21     
===========================================
+ Hits          9535     9772     +237     
- Misses           7        8       +1     
  Partials        11       11              

Flag	Coverage Δ
tests	99.80% <99.80%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/tox/tox_env/python/pip/pip_install.py	100.00% <ø> (ø)
.../tox_env/python/virtual_env/test_virtualenv_api.py	100.00% <ø> (ø)
src/tox/tox_env/package.py	97.50% <87.50%> (-2.50%)	⬇️
src/tox/execute/pep517_backend.py	100.00% <100.00%> (ø)
src/tox/plugin/manager.py	100.00% <100.00%> (ø)
src/tox/pytest.py	100.00% <100.00%> (ø)
src/tox/session/cmd/run/single.py	100.00% <100.00%> (ø)
src/tox/session/state.py	100.00% <100.00%> (ø)
src/tox/tox_env/api.py	100.00% <100.00%> (ø)
src/tox/tox_env/python/package.py	100.00% <100.00%> (ø)
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d448d82...243f238. Read the comment docs.