Get rid of OpenCV dependency as much as possible

[iitaku]

• Remove OpenCV link dependency from the image-io bb
  • Use DynamicModule and symbol loading instead of header include and link library
  • Use OpenCV C-API instead of C++ API for ABI compatibility
  • If the OpenCV library is not found at runtime, it can fail with the error log
• Keep OpenCV link dependency in the dnn bb because it seems to be impossible to replace with C-API

Here is the prototype branch.

[iitaku]

Resolved in #194