Combined exported functions

To clean up the external-facing surface, Get-DynamicParametersForCmdlet and Get-DynamicParametersForMockedFunction are now internal, and called via a single Get-MockDynamicParameters exported function with two parameter sets.
Added Help to exported "internal" functions

There are now three functions exported by Pester because they need to be accessible from other module scopes, but are not intended for users to call directly in test scripts.
These functions now have a brief comment-based help block which communicates this, should anyone happen to find them via something like Get-Command -Module Pester