FindOpenSP: Use pkg-config only as hints for main code path

before this change, pkg_check_modules(.. IMPORTED_TARGET GLOBAL)
is used for creating an imported target from which another imported
interface library named OpenSP::OpenSP is created. but pkg-config
does not account for all of CMake's other search behavior controls,
such as CMAKE_FIND_ROOT_PATH. neither does it export the full path
with OpenSP_LIBRARY.

after this change, the paths found by pkg-config are only used
as hints for the find_*() commands. and some cleanup are included:

* be QUIET when calling find_package(PkgConfig ..) and
  pkg_check_modules(..) as they are distracting from user's point of
  view. what matters is the output of find_package_handle_standard_args()
* parse the version and check for the existance of symbol as long as
  header path is found. because they only use header files.
* define OpenSP_LIBRARY as long as it exists. this just follows
  the convention. as OpenSP_FOUND implies a valid OpenSP_LIBRARY.
* wrap and intent multi-line command calls for better readability
* check OpenSP_FOUND before adding OpenSP::OpenSP, it's more
  idiomatic.

Fixes: #24313
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

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