Move to MEF-based or Automation registration for rules

[davidwengier]

Currently all of our rule XAML files are shipped on disk, but they're also included as resources in the DLLs, which is superfluous. We need to see if we can move to MEF Export or Automation in order to expose the rules. See here for information: https://github.com/Microsoft/VSProjectSystem/blob/master/doc/extensibility/adding_xaml_rules.md#via-mef-export

If this is possible then we can stop shipping XAML files to users.

[davidwengier]

Spike of this is here: https://github.com/davidwengier/project-system/tree/XamlRuleRegisterWithMEF

This needs a fair bit of work in CPS to complete.

• [ ] Add support to CPS for localized .xaml files registered via MEF
• [ ] Add support to CPS for multiple rules with the same name, in different contexts when registered via MEF

[drewnoakes]

6261 adds OverrideMode="Extend" to item rules *.FSharp.xaml, which isn't supported when registering rules via MEF. They would need to be flattened out as part of fixing this issue.