[WIP] GPR-972 collapse pythonSrc/ext for gpdb5

[zhrt123]

We're going to build gpdb on runway/buildweb in the future, on which we are unable to fetch submodules from outside repos. This patch helps collapse pythonsrc-ext submodules.

Previously, if the 'gpMgmt/bin/pythonSrc/ext' directory is empty, we assume that users install these packages from PyPI otherwise these packages are compiled while building gpdb.

After collapsing the pythonsrc-ext submodule, we introduced a new argument --with-pythonsrc-ext (default=no) for ./configure. If user specifies --with-pythonsrc-ext, these packages are compiled while building gpdb, otherwise they will not be compiled.