Double spaces have fallen out of favor in recent years because they no longer contribute to readability.
We have been using single spaces and editing related paragraphs for some time, but now it seems best to update the remaining instances to avoid churn in unrelated commits and to make it clearer what spacing contributors should use.
This is consistent with the way BackRest and BackRest test were renamed way back in 18fd2523.
More modules will be moving to pgBackRestDoc soon so renaming now reduces churn later.
This directory was once the home of the production Perl code but since f0ef73db this is no longer true.
Move the modules to test in most cases, except where the module is expected to be useful for the doc engine beyond the expected lifetime of the Perl test code (about a year if all goes well).
The exception is pgBackRest::Version which requires more work to migrate since it is used to track pgBackRest versions.
