Turn SCons.Variables.Variable into a dataclass

[mwichmann]

This moves the initialization into the Variable class itself, rather than being done in the _do_add method of the Variables class. We get nice printing and comparison for free via the dataclass mechanism, so the old methods are dropped from Variable.

Also correct one long-standing minor inconsistency: if a list of names for a variable is given, it is split into name and aliases, with the name not duplicated into the aliases. If only one name is given (no aliases), it was also added as the sole entry in the aliases; now it is not added to the aliases. All the SCons usages of aliases combine them back together anyway, like:

    if arg in option.aliases + [option.key,]:

So there's no need for the behavior of the one-name form, including the name in aliases. One test did need to change for this - the test of a custom help formatter function was wired to accept the old form, but only because of the way the custom formatter in the test was written, so this is a self-contained problem.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation