Support src-less invocation of py

Support src-less invocation of py_binary

With the recent addition of the #2671, I find that in most cases I can replace the py_console_script_binary with just a py_binary, e.g:

py_console_script_binary(
    name = "foo",
    deps = [...],
    pkg = "foo",
    script = "bar",
)

versus

py_binary(
    name = "foo",
    deps = [...], # assuming "foo" is in the deps
    main_module = "bar"
)

However I cannot do this, because the srcs cannot be empty here.

An alternative to this could be to use the python_interpreter introduced in #2507

interpreter_binary(
    name = "foo",
    deps = [...],
    args = ["-m", "foo"],
)