ENH: improve flexibility of make scripts

- the various make scripts now also search hierarchical sources,
  which makes it easier to organize sources. The secondary
  sub-directory is the lower-case value of the package stripped of
  trailing non-alphabet characters.

  For example, searching for ParaView-v5.9.1

    sources/
    |-- adios
    |   |-- ...
    |   |-- ADIOS2-2.7.1
    |   \-- adios-1.13.1
    |-- cgal
    |   |-- CGAL-4.12.2
    |   \-- CGAL-4.14.2
    |   ...
    |-- openmpi
    |   |-- openmpi-4.0.3
    |   \-- openmpi-4.1.1
    |-- paraview
    |   |-- ParaView-v5.6.1
    |   \-- ParaView-v5.9.1
    \-- scotch
        ...

- additional out-of-source build options

  FOAM_THIRD_PARTY_BUILDROOT :
     Replace WM_THIRD_PARTY_DIR as the root for build/ and platforms/

  FOAM_THIRD_PARTY_SOURCES :
     Alternative to WM_THIRD_PARTY_DIR/sources/

- rationalise internal package variable names for easier maintenance