This introduces a few utility functions that will be used for lookup of

packages and invocation of package managers.
matches_current_system() compares a list of system specifiers (OS/distro

name and optional version range) with the current system, returning a

boolean indicating if any of them match.
matches_version_range() compares a version with a version specifier,

returning a boolean indicating if it matches that range. It can also

optionally verify the name used in that version specifier.
execute() executes external processes, returning the results.

Unit tests pass.
Used these in testing.