Consider reducing repetitiveness in debugging log lines

[samuelkarp]

From #5 (review) (@kzys):

Manually adding ecr.fetcher... prefix looks error-prone. How about either 1) using SetReportCaller or 2) adding a private method on ecrFetcher that returns a logger with a prefix. The prefix can be WithField("package", "ecr.fetcher...")

SetReportCaller adds a fair amount of overhead, so that's probably not the right answer for this library. The other approach might be worth pursuing.