Cleans up a whole bunch of things I wanted to get out of the door
right away:
* depot internal references to //third_party/nixery have been replaced
with //tools/nixery
* cleaned up files from Github
* fixed SPDX & Copyright headers
* code formatting and inclusion in //tools/depotfmt checks
Change-Id: Iea79f0fdf3aa04f71741d4f4032f88605ae415bb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5486
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: tazjin <tazjin@tvl.su>
Previously, this was failing as follows:
```
these derivations will be built:
/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv
building '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv'...
building
warning: GOPATH set to GOROOT (/nix/store/4859cp1v7zqcqh43jkqsayl4wrz3g6hp-go-1.13.4/share/go) has no effect
failed to initialize build cache at /homeless-shelter/.cache/go-build: mkdir /homeless-shelter: permission denied
builder for '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv' failed with exit code 1
error: build of '/nix/store/7rbrf06phkiyz31dwpq88x920zjhnw0c-nixery-popcount.drv' failed
```
