Make consistent the expectation around non-existent fragments directory

[jaraco]

Description

In the first commit, I update the documentation to reflect the current behavior, highlighting that a missing fragments directory is normal for a "create" operation and caused by the "build" operation.

In the second commit, I expand the documentation to include the case where "build" is invoked and the fragments directory is missing to be consistent with an empty directory or no directory at all. I also draft a likely way to achieve this behavior. Once tests pass, this approach closes #538.

Note that this change aligns the behavior with the currently prescribed expectation.

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.
• If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

Looks very good. Many thanks.

Only minor comments.

[adiroiban]

Thanks. It looks like this was a bug.

The test docstring states that it is expected to handle it as no changes, but in fact it was failing :)

[adiroiban]

Suggested change

	By default, the processed news fragments are removed using ``git``, which will
	also remove the fragments directory if now empty.
	By default, the processed news fragments are removed using ``git rm``.

I don't think that towncrier (or git) explicitly removes the empty dir.

[jaraco]

I tested this expectation in one of my projects. For example, you can checkout /jaraco/skeleton, which currently has one fragment, and run towncrier build 0.0.0, and it will remove the newsfragmenta dir.

[jaraco]

Here's where I did just that yesterday:

 @ work-on skeleton
 skeleton main @ towncrier build --version 0.0.0
Loading template...
Finding news fragments...
Rendering news fragments...
Writing to newsfile...
Staging newsfile...
I want to remove the following files:
/Users/jaraco/code/jaraco/skeleton/newsfragments/+drop-py37.feature.rst
Is it okay if I remove those files? [Y/n]: y
Removing news fragments...
Done!
 skeleton main @ cat NEWS.rst
0.0.0
=====

Features
--------

- Require Python 3.8 or later.
 skeleton main @ towncrier build --version 0.0.1
Loading template...
Finding news fragments...
Failed to list the news fragment files.
FileNotFoundError: [Errno 2] No such file or directory: '/Users/jaraco/code/jaraco/skeleton/newsfragments'

 skeleton main @ mkdir newsfragments
 skeleton main @ towncrier build --version 0.0.1
Loading template...
Finding news fragments...
Rendering news fragments...
Writing to newsfile...
Staging newsfile...
No news fragments to remove. Skipping!
Done!

[adiroiban]

Thanks. Sorry for the extra trouble. I have merged it.

[iliakur]

@adiroiban you're gonna laugh, but I think we ran into this situation as well. I think we're gonna change our process for other reasons in a way that this isn't a problem. However, just in case, would you mind cutting a release with this in the next few days? Thanks a bunch!!

[adiroiban]

@iliakur here is the release PR #566