Rename smarts.core.smarts

[Gamenot]

Issue

Within smarts.core referencing the smarts module will give you two potential modules: <root>.smarts and <root>.smarts.core.smarts because the <python_path>/smarts directory exists and smarts.py as a relative import for any files in the<python_path>/smarts/core directory. These both resolve to a module and can cause ambiguous behaviour. For instance smarts.__file__ can resolve to either location depending on the order of paths in the PYTHON_PATH environment variable.

Proposed solution

Rename smarts.core.smarts to smarts.core.engine.

• Move SMARTS to smarts.core.engine
• Retain smarts.core.smarts
• Update smarts.core.smarts imports to give a deprecation warning.
• Indicate the version when smarts.core.smarts should be removed.
• Update examples to use new module.

Additional context

          This caused an odd bug. It could not resolve the difference between `smarts` and `smarts.core.smarts`. I believe should change `smarts.core.smarts` to `smarts.core.engine`.

Originally posted by @Gamenot in #1828 (comment)