Member Visibility to Support System.CommandLine.NamingConventionBinder post-Beta5

[eajhnsn1]

To support the ability to fork and maintain our own System.CommandLine.NamingConventionBuilder and System.CommandLine.Hosting (as suggested in #2576 ), some types and members in System.CommandLine need to be made public.

Prior to the NamingConventionBinder project's removal, System.CommandLine was using InternalsVisibleTo to expose the following internals to System.CommandLine.NamingConventionBinder:

SymbolResult.SymbolResultTree
ArgumentResult constructor
ArgumentConverter
ArgumentConversionResult
ArgumentConversionResultType

[jonsequitur]

We should look at whether the code in those projects can be refactored to work on top of the public surface area of System.CommandLine. The InternalsVisibleTo shortcut was done in order to separate these projects while they were in the same repo, but keeping it in place for projects in separate repos will lead to issues.