Migrate over to using pathlib and subprocess.run()

[nth10sd]

This PR:

• Moves most os.path usages to pathlib
• Removes a bunch of old APIs:
  • Fix [funfuzz] Replace captureStdout with subprocess.run() #23 - captureStdout (a blocker for Python 3 support)
  • getAbsPathForAdjacentFile
  • normExpUserPath
• Refactors code involving importing resource
• Cherry-picked rev 452816f over from Remove globals and other cleanup #169
• Made handle_rm_readonly Windows-only
• Some API function names are changed to the snake_case format, mostly the ones I touched during the migration to pathlib

I had to work around an issue involving optparse and pathlib so eventually we'll have to move everything to argparse.

I added some tests but I could definitely add more. Basic tests are passing but I haven't yet tested more, and there are likely to be more bugs lurking.

This is an early version to give ample time for feedback.

[codecov-io]

Codecov Report

Merging #198 into master will increase coverage by 5.15%.
The diff coverage is 50.42%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   44.17%   49.32%   +5.15%     
==========================================
  Files          31       37       +6     
  Lines        2685     2810     +125     
==========================================
+ Hits         1186     1386     +200     
+ Misses       1499     1424      -75

Impacted Files	Coverage Δ
src/funfuzz/js/js_interesting.py	21.42% <10.52%> (-1.03%)	⬇️
src/funfuzz/util/s3cache.py	54.23% <100%> (ø)	⬆️
src/funfuzz/js/__init__.py	100% <100%> (ø)	⬆️
src/funfuzz/ccoverage/get_build.py	100% <100%> (ø)	⬆️
src/funfuzz/js/shell_flags.py	89.93% <100%> (-0.07%)	⬇️
tests/util/test_fork_join.py	100% <100%> (ø)
tests/js/test_link_fuzzer.py	100% <100%> (ø)
tests/util/test_sm_compile_helpers.py	100% <100%> (ø)
tests/util/test_os_ops.py	100% <100%> (ø)
tests/util/test_hg_helpers.py	100% <100%> (ø)	⬆️
... and 30 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 35b5e09...79eb6a9. Read the comment docs.