Allow lazy creation of the DSOs list, which
can avoid unnecessary (~slow) probing work
when running eg. './setup.py sdist'.

Helps with GHA, where conditional values are easier than
conditionally unset.

Maybe simplify/speed up PIP installs

More convenient to override the whole platform name,
including architecture.

* probe:
  handle strings in MSVC object files
  add toolchain inspection

This small change fixes a problem when a common list object is used in setup.py to declare dependencies for multiple extension objects. In this case the library dependencies get accumulated in the same list which causes problems in more complex builds.

Since _dsoinfo.py is "standard" convention,
only allow specific override (or omission).

Generates an info module per DSO built with information on the original
dot separated name, the stem and full path to the DSO. The info module
also adds the directory containing the DSO to the DLL path on Windows
once the info module is loaded.

The name for the info module is configurable per DSO and defaults to
`"{stem}_dso.py"`. Setting the name to `None` turns off info moduel
generation for the DSO.

The change does not attempt to create proper package structure to ensure
the info modules are importable, that is up to the user.