This commit significantly improves dependency resolution (i.e., runtime
validation of whether optional and mandatory application
dependencies satisfy application requirements) by manually validating
dependency versions before deferring to increasingly unreliable
setuptools-specific logic for doing so. Specifically, this commit:

* API generalized:
  * Generalized the "betse.lib.setuptools.setuptool" submodule:
    * Refactored the die_unless_requirement() validator and
      is_requirement() tester to manually validate dependency versions
      before deferring to setuptools-specific logic for doing so.
    * Revised documentation for the
      get_requirement_distribution_or_none() getter to note that this
      function should only be called as a latch-ditch fallback.

Unrelatedly, this commit also bumps all copyright notices embedded
throughout the codebase. (Deplorable depredations of predatory statutes!)