bug#42449: texlive: l3regex.sty not shipped with texlive-latex-l3kernel

[Jonathan Brielmaier]

I'm using a latex "theme" called moderncv
(https://github.com/xdanaux/moderncv).

In `moderncv.cls` it states a dependency on l3regex:
```
\RequirePackage{l3regex}
```

But I couldn't find any texlive package providing `l3regex.sty`.
According to https://www.ctan.org/pkg/l3regex it should be included in
`texlive-latex-l3kernel`.

But something is wrong with the build:
```
find $(guix build --source texlive-latex-l3kernel) -name "*regex*"
/gnu/store/46sm302ggnmk5hcr08psiazj24svpp5d-texlive-latex-l3kernel-51265-checkout/l3regex.dtx

find $(guix build texlive-latex-l3kernel) -name "*regex*"
[no result].
```