Clean up loaddata

[William Schwartz]

In django.core.management.commands.Command, load_label is 66 lines long with maximum indentation of 9 levels (the row starts at column 37). find_fixtures is 56 lines. These monolith methods are hard to read, hard to override, and, in some places, violate ​PEP-8's recommendation to keep try blocks small.

The reason I care about this is that an app I'm working on may need to customize how loaddata finds fixtures, and, unlike templates, there isn't a nice loader API to hook into. So that leaves me with overriding find_fixtures. My initial attempt was a mess because of how giant the parent class's method is.

I am submitting ​PR GH-13842 with my proposed refactoring. The commits will need to be squashed before merging the PR, but I wanted to include commit message to justify some of the less obvious-looking changes.

[Alexandr Tatarinov]

Looks good to me, awesome work!

[William Schwartz]

Thanks, Alexandr. Would you be able to review the PR?

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1e655d35:

Refs #32317 -- Cleaned up try/except blocks in loaddata command.

This moves code unable to trigger relevant exceptions outside of
try/except blocks, and changes 'objects' to 'objects_in_fixture'
which is equal to the length of 'objects'.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1557778:

Refs #32317 -- Simplified find_fixtures() in loaddata command.

This always replaces 'fixture_name' with its base name, which preserves
the previous behavior, because os.path.basename() was not called only on
relative paths without os.path.sep i.e. when base name was equal to the
file name.

This also changes os.path.dirname() and os.path.basename() calls to the
equivalent os.path.split() call.

[Mariusz Felisiak <felisiak.mariusz@…>]

In de32fe83:

Fixed #32317 -- Refactored loaddata command to make it extensible.

Moved deeply nested blocks out of inner loops to improve readability
and maintainability.

Thanks to Mariusz Felisiak, Shreyas Ravi, and Paolo Melchiorre for
feedback.