Do not restrict CMAKE_MODULE_PATH to a single path

[vaavaav]

Right now, braft's CMakeLists.txt restricts CMAKE_MODULE_PATH to ${PROJECT_SOURCE_DIR}/cmake. However, this makes it harder to link braft against other libraries, since developers will have to copy Find<library>.cmake files to that path, which is impractical.

Instead, allow the user to provide other paths while still maintaining this one by changing that line to: list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake").

With this, developers can provide other paths where Find<library>.cmake files exist in the command line (for example, with cmake -DCMAKE_MODULE_PATH=opt/lib/cmake).