This speeds up `./mach build --dev` followed by `./mach build-cef` by
25%. When rust-lang/cargo#497 is fixed, this speedup will increase
dramatically.
Source-Repo: https://github.com/servo/servo
Source-Revision: d6263c9b6e969fde4c644034e684a39d68667ad9
Fixes#6236
Also included in this commit are the changes need to make flake8 pass
for the existing python file
Source-Repo: https://github.com/servo/servo
Source-Revision: ccfe29d8f284dedc0101045d574a98fb1f69aa62
