Revise semantics of compatibility rule profile inclusion

UseCompatibleCommands and UseCompatibleTypes create a "union" profile for comparison of target profiles to a base standard. This allows us to use simple heuristic to determine whether a PowerShell AST represents something that is platform-specific (we see if the command or type is available in any platform, and if not ignore it as user-defined).

The problem here is that to create good warnings we have to have a good baseline union profile. Meaning we need to be judicious about how we make the profile. But since we could get it wrong, I created an informal mechanism for including things into the union profile. We use all the profiles in the "profile directory" and nothing else:

https://github.com/PowerShell/PSScriptAnalyzer/blob/df566b577803375cd6f1faba7d9d5d8715deefab/PSCompatibilityAnalyzer/Microsoft.PowerShell.CrossCompatibility/Utility/CompatibilityProfileLoader.cs#L76

This means that:

• Including only one target, you still get good warnings, because it's compared against all the other profiles we ship by default
• Using a target not in the profile directory, that won't be included in the union comparison
• You can configure a different profile directory to get rid of comparisons to PowerShells you don't care about

This works but it's a bit subtle. I think we should move to:

• All targets are in the union by default
• Targets in the profile directory are in the union by default
• Targets can be excluded from the union in a configuration setting, by name or path
• The profile directory automatic union inclusion can be turned off by default

從 CompatibilityProfileLoader.cs 開始，尤其是 issue 中連結的設定檔目錄載入邏輯，並追蹤 UseCompatibleCommands 和 UseCompatibleTypes 如何建構聯合設定檔。定義對具名目標或以路徑為基礎的目標，以及停用自動包含設定檔目錄的設定與排除行為。完成條件是預設包含所有目標，同時遵循已設定的排除項目和退出設定。