Move python detection into separate function call

[rarkins]

Today python and pipenv are combined together, but we should separate out the python part. Although there might be common files parsed in some instances, the separation is better because we need to support other tools like pip, pip-compile, poetry, uv, etc - and there will be a lot of duplication if they all reimplement detection in files like .python-version.

[viceice]

• Support autodetect install-tool python containerbase/base#4441