As a small convenience, to allow us to run tests for an invididual
cmdlet when doing development testing.

This command replaces the Get-ChecksumValid helper function, adding an
alias for compatibility purposes.

Add tests to ensure the behaviour of rewritten command matches the
original behaviour.

When working on this, some irregularities were noted with the generation
of the documentation, so these changes avoid the issues that arose.

This allows us to maintain a simple list of deprecated aliases and the
new command name, or commands that are being removed, and automatically
warn when that command is called.