Support both local installation and hermetic build for Python.

[zhe-tao]

Reasons for those changes

1. Why setup the @python repository: Python projects with C/C++ extensions/modules require corresponding cpython headers and libraries.
2. Why add options to build a hermetic Python: setting up a local bazel_python installation is efficient but may confuse new users like the reviewers.
3. BTW, we want to fix Fail the execution if the specified bazel_python version is not installed. #14.

Changes in bazel_python.bzl

bazel_local_python

bazel_local_python replaces the original bazel_python. Changes:

1. It now takes new python_version and name='python' arguments.
2. It will create a local repository @name (@python by default) and expose cpython headers, libraries (@python or @python//:python) and binaries (e.g., @python//:bin/python3).

bazel_hermetic_python

The new bazel_hermetic_python fetches a python source code and builds it using @bazel_python//:external/python*.*.BUILD. It has the same interface as bazel_local_python and gives the build @python repository.

bazel_python_venv

bazel_python_venv no longer takes the python_version argument. Instead, it takes an (implicit) python (tool) argument to specify which Python repository and binary to use. It by default is @python//:bin/python3.

[adityathakur]

Know --> Known

[adityathakur]

Should README.md also be updated?