FindOpenSP: Use pkg-config only as hints for main code path
This module was added by !7470 (merged), but the concern raised by the author in !7470 (comment 1232445) was never fully resolved. Discussion in !8037 (closed) identified that it is the same problem as #24028 (comment 1255941): pkg-config does not account for all of CMake's other search behavior controls, such as CMAKE_FIND_ROOT_PATH
.
The FindOpenSP module needs to be updated to only use pkg-config as hints for the main find_*
code path. Then the results need to be used to construct an imported target. That way we present results as absolute paths consistently whether or not pkg-config was used. Several other find modules follow this pattern already.