split out package validation so that it can be used on server.

The validation code is no longer in ExtensionManagerDomain itself.
updated version number
eliminate errors that are unused in the ExtensionManagerDomain
Allows validation with required package.json.

For the extension registry, we want package.json to be required.
With this change, the validation code allows optional checking for package.json.
Merge pull request #3261 from adobe/dangoor/shared-extensibility

Breaks out package validation for sharing with brackets-registry