Add calloop::sources=debug to default log filters

[BigWingBeat]

Objective

When setting the default log level to trace (e.g. with RUST_LOG=trace), one of the log messages that gets enabled looks like [calloop] Processing events for source type .... This message is logged at the trace level 1-3 times every frame, making it extremely spammy. Although the purpose of trace-level logs is to be verbose, logging the same unactionable thing every frame is excessive.
While normally there would be a bunch of stuff that always gets logged every frame, the default filters provided by LogPlugin exclude all of that at every log level, except for this one thing which slips through.

Solution

Add the offending module to the default filters.

Testing

Manually tested with the logs example and various RUST_LOG values.