fix: building the package’s dist files should *always* generate the build’s epoch as well, even when using the individual package goals only

[jenstroeger]

The issue was that we can build an artifact but we won’t have the artifact’s build epoch timestamp:

> make dist/package-2.16.0.tar.gz
...

SOURCE_DATE_EPOCH=1748988170 flit build --setup-py --format sdist
Found 36 files tracked in git                                                                                      I-flit.sdist
Writing generated setup.py                                                                                         I-flit.sdist
Built sdist: dist/package-2.16.0.tar.gz                                                                       I-flit_core.sdist
> ll dist/
total 24K
-rw-r--r-- 1 savage admin 24K Jun  4 08:03 package-2.16.0.tar.gz

This change fixes that issue:

> make dist/package-2.16.0.tar.gz
...

echo 1748988267 > dist/package-2.16.0-build-epoch.txt
SOURCE_DATE_EPOCH=1748988267 flit build --setup-py --format sdist
Found 36 files tracked in git                                                                                      I-flit.sdist
Writing generated setup.py                                                                                         I-flit.sdist
Built sdist: dist/package-2.16.0.tar.gz                                                                       I-flit_core.sdist
> ll dist
total 28K
-rw-r--r-- 1 savage admin  11 Jun  4 08:04 package-2.16.0-build-epoch.txt
-rw-r--r-- 1 savage admin 24K Jun  4 08:04 package-2.16.0.tar.gz
> cat dist/package-2.16.0-build-epoch.txt 
1748988267

We could probably make this more readable by using a dedicated build-epoch goal or some such… 🤔

This actually becomes useful in combination with PR #947 so that we can now use:

> make simple-index

which builds the packages, their epoch, and the pip Simple Repository.

[behnazh]

I'm a bit confused because in

dist: dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-py3-none-any.whl dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION).tar.gz dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-docs-html.zip dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-docs-md.zip dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-build-epoch.txt

we already add the dist/$(PACKAGE_NAME)-$(PACKAGE_VERSION)-build-epoch.txt dependency. Why do we need to add that again for each dist target?

[jenstroeger]

Because

make dist/package-v2.16.0-py3-none-any.whl

or

make dist/package-v2.16.0.tar.gz

produce the package artifacts but no epoch file, so that we end up with the package but we can’t reproduce it because we don’t know when that particular package file was created.