This commit updates the build system to follow the established CMake
convention.

It also allows the use of iMSTK custom find package in the context of
external project named after the library being integrated and avoid
target name conflicts.

References:

* Double colon in target name means ALIAS or IMPORTED target.

  Source: https://cmake.org/cmake/help/latest/policy/CMP0028.html

* This convention of double-colons gives CMake a hint that the name
  is an IMPORTED target when it is used by downstreams with the
  target_link_libraries() command. This way, CMake can issue a
  diagnostic if the package providing it has not yet been found.

  Source: https://cmake.org/cmake/help/latest/manual/cmake-packages.7.html