Fix get message from resource of basename with dots and refactor license ResourceBundleMessageSourceTests

Prior to this commit, doesn't get message from ReloadableResourceBundleMessageSource if basename exist dots but slashes, for example "package.name.messages.properties". Following that change, these names are valid for getting messages

Issue: SPR-5503

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Comment From: snicoll

thanks for the PR and sorry it took so long to process it. There hasn't been a lot of traction from the community and the related issue has been closed as a result. Besides, the difference of behavior with ResourceBundleMessageSource is actually documented. Thanks anyway.