Clarify entry point's object reference specification

[McSinyx]

Issue Description

In the entry point specification, section data model,

The object reference points to a Python object. It is either in the form importable.module, or importable.module:object.attr. Each of the parts delimited by dots and the colon is a valid Python identifier. It is intended to be looked up like this:

import importlib
modname, qualname_separator, qualname = object_ref.partition(':')
obj = importlib.import_module(modname)
if qualname_separator:
    for attr in qualname.split('.'):
        obj = getattr(obj, attr)

First, I propose to disallow the former form importable.module as its invocation is never mentioned.

Second and more importantly, I want to explicitly enforces object on the right of the colon (:) to be an attribute of importable.module on the left. To add the confusion , later in the section use for scripts:

the entry point mycmd = mymod:main would create a command mycmd launching a script like this:

import sys
from mymod import main
sys.exit(main())

Because of the way the import-from statement works:

The from form uses a slightly more complex process:

1. find the module specified in the from clause, loading and initializing it if necessary;
2. for each of the identifiers specified in the import clauses:
   1. check if the imported module has an attribute by that name
   2. if not, attempt to import a submodule with that name and then check the imported module again for that attribute
      [...]

package authors may wrongly assume the first identifier object on the right of the colon to possibly be a module as well (exhibit A). While both pkg_resources's and the builtin importlib.metadata's EntryPoint.load would fail on such incorrect entry point, they aren't run on common build processes and the most popular installer pip generates working scripts in a format the same as in the mycmd = mymod:main example.

It would be nice if the specification is more explicit over what is and is not allowed instead of a reference implementation ([an object reference] is intended to be looked up like this). In Guix we got entry point script generation (hopefully) right on the third try and it shouldn't be like this for other projects.

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