Due to inlining it is possible to visit the same module multiple times
during `<Cache as DocFolder>::fold_crate`, so we keep track of the
modules we've already visited.
The script is intended as a tool for doing every sort of verifications
amenable to Rustdoc's HTML output. For example, link checkers would go
to this script. It already parses HTML into a document tree form (with
a slight caveat), so future tests can make use of it.
As an example, relevant `rustdoc-*` run-make tests have been updated
to use `htmldocck.py` and got their `verify.sh` removed. In the future
they may go to a dedicated directory with htmldocck running by default.
The detailed explanation of test scripts is provided as a docstring of
htmldocck.
cc #19723
