scripts: move python modules to python directory

Up until now, importing an SPDK RPC python module was just a matter of
`import rpc`. It's fine until there's another module called `rpc`
installed o

scripts: move python modules to python directory

Up until now, importing an SPDK RPC python module was just a matter of
`import rpc`. It's fine until there's another module called `rpc`
installed on the system, in which case it's impossible to import both of
them. Therefore, to avoid this problem, all of the modules were moved
to a separate directory under the "spdk" namespace.

The decision to move to a location under a separate directory was
motivated by the fact that a directory called scripts/spdk would look
pretty confusing. Moreover, it should make it also easier to package
these scripts as a python package.

Other than moving the packages, all of the imports were updated to
reflect these changes. Files under python now use relative imports,
while those under scripts/ use the "spdk" namespace and have their
PYTHONPATH extended with python directory.

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Change-Id: Ib43dee73921d590a551dd83885e22870e72451cf
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9692
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

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