make stubgen generate PEP-561/typing docs compliant '-stubs' package directory names

[froh]

Bug Report

the stubgen --output doesn't generate package directories named as mandated by PEP-561/the typing docs.

PEP-561 states that "The name of the stub package MUST follow the scheme foopkg-stubs for type stubs for the package named foopkg."

To Reproduce and Actual Behavior

currently the packages in are placed into subdirectories named like the package.
so for a package foobar, the stubs currently are generated into a out/foopkg directory, where out is the default for the --output parameter to stubgen, like so:

$ stubgen --inspect-mode --ignore-errors --package mypy
... lots of output
$ find out -type d
out/
out/mypy
out/mypy/server
out/mypy/plugins
out/mypy/dmypy

Expected Behavior

the output should instead have a -stubs suffix to the üpackage name, out/foopkg-stubs, like so:

$ stubgen --inspect-mode --ignore-errors --package mypy
... lots of output
$ find out -type d
out/
out/mypy-stubs
out/mypy-stubs/server
out/mypy-stubs/plugins
out/mypy-stubs/dmypy

Your Environment

$ pip list
Package           Version
----------------- -------
mypy              1.11.2
mypy-extensions   1.0.0
$ python --version
Python 3.12.6