Add a "requires" directive, extend functionality of package requirements (#36286)

Add a "require" directive to packages, which functions exactly like
requirements specified in packages.yaml (uses the same fact-generation
logic); update both to allow making the requirement conditional.

* Packages may now use "require" to add constraints. This can be useful
  for something like "require(%gcc)" (where before we had to add a
  conflict for every compiler except gcc).
* Requirements (in packages.yaml or in a "require" directive) can be
  conditional on a spec, e.g. "require(%gcc, when=@1.0.0)" (version
  1.0.0 can only build with gcc).
* Requirements may include a message which clarifies why they are needed.
  The concretizer assigns a high priority to errors which generate these
  messages (in particular over errors for unsatisfied requirements that
  do not produce messages, but also over a number of more-generic
  errors).

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