Bug: auto-loaded `last_updated_by_git` causes failure in non-git environments

[tang-mm]

Description

canonical-sphinx auto-enables the sphinx_last_updated_by_git package if it's present in the venv, even when it's not added to the user extension list. When running build in non-git environments, this causes build warnings. For doc projects with --failed-on-warning enabled, it results in build failures.

This could happen in:

• builds in a Docker container without .git directory
• builds from the source tarball

Expected Behavior

User settings in conf.py be able to prevent sphinx_last_updated_by_git from loading and running.

Cause

When using sphinx-sitemap extension, it installs sphinx_last_updated_by_git as a hard dependency and provides the sitemap_show_lastmod flag in conf.py to toggle the features.

However, because canonical-sphinx detects that the package is installed in the venv (config.py), it auto-loads the package during doc build even if it's not added as a project extension in conf.py. This behavior forces Sphinx builds to check for git history, and cannot be disabled by setting sitemap_show_lastmod=False.

Steps to Reproduce

1. Create a starter-pack project without .git directory (e.g. by renaming the .git or copying the docs/ dir outside the git project)
2. Remove sphinx_last_updated_by_git from the requirement list and extension list in conf.py, but keep sphinx-sitemap.
3. Set sitemap_show_lastmod=False in conf.py
4. Re-run make install and make html

Output:

getting Git timestamps for source files... [100%] .
getting Git timestamps for dependencies... [100%] 

WARNING: Error getting data from Git (no "last updated" dates will be shown for source files from <path>/docs/explanation):
fatal: not a git repository (or any of the parent directories): .git