Improve error message for missing packages when installed via git URL in requirements.txt

[hmedina]

Describe the bug

I have a document that specifies in its requirements.txt file a git+https link to a package; this gets installed under src/. If I remove that folder (e.g. to install the package in "editable form" elsewhere), a ModuleNotFoundError error appears, however it does not specify what module is missing. I'm reporting this as "Please also report this if it was a user error, so that a better error message can be provided next time."

How to Reproduce

Assuming the requirements.txt file contains something like

pygments >= 2.7.1, < 3
Pygments_Kappa_plugin @ git+https://github.com/hmedina/Pygments_Kappa_plugin.git

The package Pygments_Kappa_plugin will get installed under src/Pygments_Kappa_plugin. If I delete that folder, make clean & make html, I get:

$ make html
Running Sphinx v8.2.3
loading translations [en]... done
making output directory... done
checking bibtex cache... out of date
parsing bibtex file /home/hmedina/project/bibliography.bib... parsed 70 entries


Versions
========

* Platform:         linux; (Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.39)
* Python version:   3.13.2 (CPython)
* Sphinx version:   8.2.3
* Docutils version: 0.21.2
* Jinja2 version:   3.1.6
* Pygments version: 2.19.1

Last Messages
=============

None.

Loaded Extensions
=================

None.

Traceback
=========

      File "<frozen importlib._bootstrap>", line 1324, in _find_and_load_unlocked
    ModuleNotFoundError: No module named 'core'


The full traceback has been saved in:
/tmp/sphinx-err-clglaqti.log

To report this error to the developers, please open an issue at <https://github.com/sphinx-doc/sphinx/issues/>. Thanks!
Please also report this if it was a user error, so that a better error message can be provided next time.
make: *** [Makefile:20: html] Error 2

A better error message would mention that a package, Pygments_Kappa_plugin, is now missing, instead of only mentioning core.

Environment Information

Platform:              linux; (Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.39)
Python version:        3.13.2 (main, Feb 12 2025, 04:07:04) [GCC 13.3.0])
Python implementation: CPython
Sphinx version:        8.2.3
Docutils version:      0.21.2
Jinja2 version:        3.1.6
Pygments version:      2.19.1

Sphinx extensions

Additional context

No response