Why is `openin_any = p` restricting access to packages in TEXMFDIST?

[Adrian]

Hi,

since recent texlive 2020 versions (not sure when exactly, but I did not have
the problem with texlive 2019 for sure) `openin_any = p` started restricting
access to packages in TEXMFDIST as well:

I'm using `openin_any = p` (set in a custom `TEXMFCNF`) as an additional line of
defense in an application that runs on (partially) user-provided LaTeX code.

Now I'm getting this error when building pretty much any document that uses
packages:

```
/opt/texlive/bin/x86_64-linux/xelatex: Not reading from /opt/texlive/texmf-dist/tex/latex/l3packages/xparse/xparse-generic.tex (openin_any = p).
! LaTeX3 Error: File 'xparse-generic.tex' not found.
```

>From what I understood when googling, even with `openin_any = p` files in
`TEXMFDIST` are supposed to be fine. Is this a bug, or is this an intentional
change. In any case, I'm looking for a solution that allows access to anything
in `TEXMFDIST` in addition to the directory containing my tex file.

According to my TeX Stack Exchange question [1], a minimal document triggering the
error is \documentclass{article}\usepackage{xparse}\begin{document}\end{document}

[1] https://tex.stackexchange.com/q/563920/1651

Cheers
Adrian