gh-149010: Improve reliability of inspect CLI

[ncoghlan]

• Handle non-source modules more gracefully (and consistently)
• Improve handling of frozen modules (which may or may not have source)
• Avoid reporting misleading info when looking up objects via aliases
• Refactor CLI implementation to improve testability
• Add several more test cases

Closes #149010

• Issue: Improve inspect module CLI handling of non-source modules #149010

[ncoghlan]

Some before-and-after examples.

Before:

acoghlan@TerminalMist:~/devel/cpython$ python3 -m inspect sys --details
Can't get info for builtin modules.

After:

$ ./python -m inspect sys --details
Target: sys
Origin: built-in
Source: None
Cached: None
Loader: <class '_frozen_importlib.BuiltinImporter'>

Before:

$ python3 -m inspect _pickle --details
Target: _pickle
Origin: None
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/lib64/python3.13/inspect.py", line 3460, in <module>
    _main()
    ~~~~~^^
  File "/usr/lib64/python3.13/inspect.py", line 3441, in _main
    print('Cached: {}'.format(module.__cached__))
                              ^^^^^^^^^^^^^^^^^
AttributeError: module '_pickle' has no attribute '__cached__'

After:

$ ./python -m inspect _pickle --details
Target: _pickle
Origin: /home/acoghlan/devel/cpython/build/lib.linux-x86_64-3.15/_pickle.cpython-315-x86_64-linux-gnu.so
Source: None
Cached: None
Loader: <_frozen_importlib_external.ExtensionFileLoader object at 0x7e9d38f2a2c0>

Before:

$ python3 -m inspect os:path.join --details
Target: os:path.join
Origin: /usr/lib64/python3.13/os.py
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/lib64/python3.13/inspect.py", line 3460, in <module>
    _main()
    ~~~~~^^
  File "/usr/lib64/python3.13/inspect.py", line 3441, in _main
    print('Cached: {}'.format(module.__cached__))
                              ^^^^^^^^^^^^^^^^^
AttributeError: module 'os' has no attribute '__cached__'

After:

$ ./python -m inspect os:path.join --details
Target: posixpath:join (looked up as "os:path.join")
Origin: frozen
Source: /home/acoghlan/devel/cpython/Lib/posixpath.py
Cached: None
Line: 72

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32526303 | 📁 Comparing d94403b against main (1504bd6)

  🔍 Preview build  

2 files changed

± library/inspect.html
± whatsnew/changelog.html

[hugovk]

It's okay to put this at the top for tests. Can even lazy import re if you want.

There's also one in test_re_module_has_signatures we could move.

[ncoghlan]

Given how long the CI takes to run, I'm going to put that on the "try to remember to clean that up next time I touch this module" list :)