bt2: Rename files to use singular form

I am mildly annoyed that most Python files use the singular form of the
concept they are related too (component.py, event.py) but others use the
plural form (fields.py, values.py).

Change this to consistently use the singular.

Change-Id: I1a33c47ec45ef3359025e0a66f01eaefc09fecdf
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.gerrithub.io/c/eepp/babeltrace/+/452257
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: Philippe Proulx <eeppeliteloop@gmail.com>