[base]: default_variants could check variant conflicts

[RJVB (René Bertin)]

Consider a port foo with

variant A conflicts B description {AAA} {}
variant B conflicts A description {BBB} {}
default_variants +A

> port info foo +B
Error: fpp: Variant A conflicts with B
Error: Unable to open port: Error evaluating variants
Exit 1

I'd argue that it isn't really too much to expect that default_variants would check check the user variant selection before applying the default; most APIs that I know of that prevent a default do something that boils down to the same thing. Note that I'm not saying it's easy, but it would remove (some of) the burden for conflict management and determining whether or not to set a default.