CLI: Show correct help

[bricelam]

The general approach here is to copy the command definitions from ef into dotnet-ef so that when --help is specified (or it's a known help-only command like dotnet ef, dotnet ef migrations, etc.) we show the help without dispatching to ef. It also ensures the ef-specific options (like --assembly) are replaced by the dotnet-ef ones (like --project).

Fixes dotnet/efcore#6592

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