PR #208478 added a lot of documentation about which packages were
rebuilt in each stage of the stdenv bootstrap. However nothing
checks that these comments agree with reality; they can bitrot over
time. This PR rewrites those comments as assertions, so they cannot
bitrot.
This conversion did expose some ambiguity in our scheme for naming
the stages. Suppose that `pkgs.stdenv.name=="stdenv-stage4", then
which of these is "the stage4 coreutils"?
```
pkgs.coreutils
pkgs.stdenv.__bootPackages.coreutils
```
The choice is arbitrary, and both choices have confusing corner
cases. We should revisit this at some point.
d7aad24531