PackageExport: remove template Upset file

[niezbop]

The need to maintain both the packageexport.asset and the templateUpset.xml in a project is cumbersome and should be streamlined, by putting the depencies and the configuration into the asset directly.

To do this, we need to:

• Extract the custom fields we use in the UpfileEditor.cs to keep things DRY
• Introduce the dependencies and the configuration in the scriptable object

What needs to be considered:

• Backwards compatiblity: any system relying on the packageexport.asset would fail if it relies on an template Upset. Should we then accept breaking changes (after all we're still in beta), or provide some sort of migration mechanism, perhaps relying on an Asset Processor?