if another python3 with higher version is found by
find_package(Python3), the cmake's install script would just
install the python modules/extensions into that python3's
dist-package directory, and the packaging script would fail
to find these artifacts when trying to package them.

so we need to ensure that the install directories for python
modeules/extensions are always "versioned" with WITH_PYTHON3
cmake option.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit e2babdf)

* CMakeLists.txt:
    always pass "EXACT" to find_package(Python3).
    because per cmake document, "EXACT" only takes effect when
    <Package>_FIND_VERSION_COUNT is greater than 1, where <Package>
    is "Python3". see also cmake/modules/FindPython/Support.cmake
* cmake/modules/AddCephTest.cmake:
    drop redundant find_package(Python3) calls. since Python3 is
    a mandatory requirement for building Ceph, we only need a
    single call of find_package(Python3..) in the top of the source
    tree. the only possible case to repeat it is to ensure that we
    have the correct version of Python3 used in following CMake
    script. but there is no need to repeat it if we just want to
    ensure that we have a python3 interpretor in place.
* cmake/modules/Distutils.cmake:
    always pass "EXACT" to find_package(Python3).
    we should always pass EXACT to find_package() when finding python3,
    this is a follow-up of e2babdf

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit ea4ae6d)

Conflicts:
	cmake/modules/AddCephTest.cmake: trivial resolutions