[DX] Support a way to declare conflicts in .info files

This idea first popped up with two contrib modules that use the same database table (from D7), but might also be helpful regarding Forum version 1.x-1.x and Forum version 1.x-2.x, basically any modules that compete for the same resources.

A module should be able to declare a conflict in an easy way. This currently has to be done with custom code in hook_requirements() - and that possibly has to happen in both conflicting modules.

It would be much easier, if that could be done in the info file, similar to the way dependencies are declared.

• In the case of modules that compete for the same resources, it could be:

  name = Foo Module
  description = Does footastic things
  type = module
  backdrop = 1.x
  
  dependencies[] = needed_module (<1.12)
  conflicts[] = other_foo_module
  

  If one of the two conflicting modules is already installed, the installer should refuse to enable the other one. No matter, which of the two modules declares the "conflict".

• In the case of conflict or incompatibility between different major versions of the same module (where for example no upgrade path is ready yet between v1.x and v2.x):

  v2.x could declare:

  name = Forum
  description = Provides discussion forums.
  type = module
  backdrop = 1.x
  
  dependencies[] = views
  conflicts[] = forum (<2.0)
  

  v1.x could declare

  name = Forum
  description = Provides discussion forums.
  type = module
  backdrop = 1.x
  
  conflicts[] = forum (>=2.0)
  

  In this case:

  • on sites where v1 of the module is installed, v2 of the module should NOT be offered as an option for upgrade
  • on new sites, v2 of the module should be provided as the default/recommended version

Does this seem feasible?