pwkit/cli/latexdriver.py: try to straighten out the BibTeX situation
pwkit/cli/latexdriver.py: try to straighten out the BibTeX situation

I want a BibTeX solution that lets me use Git to collaborate with other folks
on manuscripts. I also want to use my bibtools to automatically generate my
BibTeX files. Unfortunately, the `-e` option to `latexdriver` was a no-go
since no one else has bibtools installed (for now).

I am also not a fan of having people commit their random .bib files to the
repository since their random .bib files are, generally, full of junk and will
lead to duplicate entries, etc.

Therefore, we add a quick system to fill in the "main" BibTeX file by merging
other BibTeX files that, importantly, are not kept in version control. This
ensures that the BibTeX database is always minimal and normalized.

We also add some glue to optionally feed in bibtools data to this merging
system. The "optional" part is essential so that the same settings will also
work for someone who doesn't use bibtools.

I *think* this is the right path forward. There's still opportunity for
duplicated entries, inconsistent journal naming schemes, etc., but that can be
fixed up by editing the input .tex files and the master .bib file manually.
Solving those problems in code, in full generality, is probably impossible.

pkgw/pwkit