Support custom base profiles in MigrationTool without subclassing

[ericof]

Distributions can support having a base profile other than Products.CMFPlone:plone, allowing for more granular control over what gets installed—including dependencies—during site creation.

Currently, to fully benefit from this approach, distribution maintainers must subclass Products.CMFPlone.MigrationTool.MigrationTool, as demonstrated here in portalbrasil.core.

A more flexible alternative would be to allow setting the package name (e.g., portalbrasil.core) and the base profile (e.g., portalbrasil.core:base) as persistent attributes on the existing MigrationTool. This would remove the need for subclassing and simplify adoption for other distributions.