[PEP] Evaluate / rework `pyomo.environ`

[mrmundt]

Summary

We currently import a lot of symbols into pyomo.environ and then recommend to folks that they do import pyomo.environ as pyo. We aren't sure if this is what we should do moving forward.

Rationale

There were some choices that made sense in the past about namespaces that may not make sense anymore.

Description

• All components promoted to "pyomo core" should be imported in pyomo.environ
  • DAE, GDP, etc.
  • Do an audit of pyomo.environ as part of this
  • "from pyomo.environ import DerivativeVar" vs. "from pyomo.environ.dae import DerivativeVar" vs. "import pyomo.environ as pyo; m.d = pyo.dae.DerivativeVar()"
• Get rid of "pyomo.environ" entirely?? Auto register the plugins when pyomo is imported. "import pyomo as pyo" and leave pyomo.environ as an alias