opencv/opencv4: eliminate portfile duplication, via subport

[mascguy (Christopher Nielsen)]

Now that the two OpenCV-related ports have been brought up to parity, there are very few differences between them. Eliminate the portfile duplication, via subports.

Also add the following variants, which I missed when bringing both up to parity:

• opencv: openmp
• opencv4: openni

Other notable cleanup that will be tackled during the reconciliation:

• Universal use of github portgroup. Currently used for opencv4, but not for opencv/opencv3.
• Adopt cmake portgroup, to reduce number of CMake flags needed.
• Fully implement variant openmp, which is effectively a stub at this point. (While OpenMP is being enabled via configure flag, compiler.openmp_version isn't being set. With the result being that OpenMP isn't found, effectively making this variant a no-op at the moment.)

There is additional cleanup as well, but that covers the most notable items.

[mascguy (Christopher Nielsen)]

This will be done in phases, coordinated with reinstatement of OpenCV 2 port (issue:49670):

1. Add variants missed during opencv/opencv4 reconciliation (opencv +openmp, opencv4 +openni).
2. Add new port opencv3, segregated like opencv4. Also include initial changes, supporting later conversion to subport.
3. Update ports presently dependent upon opencv, to use opencv3. And update port opencv to be a stub port, with replaced_by opencv3, etc.
4. Add new port opencv2, using same segregation strategy, etc.
5. Reconcile all three into single opencv port, with three subports. This step may be combined with previous.

Thoughts/feedback welcome, as always.

[mascguy (Christopher Nielsen)]

PRs submitted for first two phases:

​https://github.com/macports/macports-ports/pull/9777

​https://github.com/macports/macports-ports/pull/9789

[Christopher Nielsen <mascguy@…>]

In 1a1ad1dde0f1ed1a99f5bf431a30fa7eeaa269d0/macports-ports (master):

opencv3: new port to replace opencv, with opencv4-style isolation

See: #62011
See: #49670

[Christopher Nielsen <mascguy@…>]

In 33d1482c8b63962c23c3d6bc4556833317fff821/macports-ports (master):

opencv: add variant openmp

See: #62011

[Christopher Nielsen <mascguy@…>]

In 49c839a35e1286a93ca495f13c268708783b4b3e/macports-ports (master):

opencv4: add variant openni

See: #62011

[Christopher Nielsen <mascguy@…>]

In 7b38728c237c1be013b382640875f03d95d30e6f/macports-ports (master):

opencv/opencv4: variant openmp: implement fully, via compiler.openmp_version

See: #62011

[Christopher Nielsen <mascguy@…>]

In 9987e0ce91de77778a17587f73061e550365cb1d/macports-ports (master):

opencv3: variant openmp: implement fully, via compiler.openmp_version

See: #62011