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.