Improved support for moved/renamed/deprecated modules

[jsiirola]

Fixes # .

Summary/Motivation:

NOTE: This PR is built on #3382. Please review / merge that PR before looking at this one. If you want to get a head start, you can compare this branch against the doc-reorg branch.

This PR replaces pyomo.common.deprecation.relocated_module() with pyomo.common.deprecation.moved_module(). The new functionality supports renamed / moved / deprecated modules through a plugin to the importlib system (instead of hacking sys.modules). This:

• is more robust
• allows us to remove stub files that have been accumulating in the codebase
• silence all deprecation warnings that are currently being emitted when building the documentation
• removes the deprecated modules from the online documentation Library Reference
• supports deprecation paths for more complex module reorganizations (e.g., converting all plugins directories into plugins.py modules)

Changes proposed in this PR:

• deprecate pyomo.common.deprecation.relocated_module()
• add pyomo.common.deprecation.moved_module().
• remove deprecated "stub" modules
• move deprecated modules that import specific functionality into the pyomo._archive directory
• rename pyomo/__future__.py -> pyomo.future.py
  • continue to support from pyomo.__future__ import ...
  • this resolves exceptions when attempting to run python within the pyomo/pyomo directory.

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