Convert module to a package directory with a py.typed file

[jwodder]

The module needs to be converted to a package directory so that it can contain a py.typed file, which is necessary so that mypy can type-check uses of duct items in the test code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.46%. Comparing base (83beb34) to head (a9ad991).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   88.42%   88.46%   +0.03%     
==========================================
  Files           1        2       +1     
  Lines         311      312       +1     
  Branches       41       41              
==========================================
+ Hits          275      276       +1     
  Misses         30       30              
  Partials        6        6              

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[asmacdo]

Looks reasonable to me, however @yarikoptic (at least when we first started) wanted the whole thing in a single python file that could be executed directly without even needing to install anything, including duct itself.

I guess we could move version back to main? This looks more idiomatic though, @yarikoptic do you still want "executable as a single file script" to be a requirement?

[yarikoptic]

So we might indeed refactor it to reside in multiple files but not sure if we are reaching threshold yet -- couldn't we substitute __version__ within __main__.py directly (if you really want to be able to python -m duct it)

[jwodder]

The biggest benefit to putting __version__ in __init__.py is that it'll show up when users do pydoc duct. Other than that, it's mostly just tradition.

[asmacdo]

I'd vote to go ahead and leave 'version' in 'init' so we don't build up weirdness to address later. If the no-install use case is requested by users maybe we revisit?

[yarikoptic]

ok