Encapsulate xprocess internals info, popen and log handles into single object for resource cleanup

[northernSage]

Following @RonnyPfannschmidt suggestion in #64, encapsulating the references and objects used for process resource clean-up in XProcess seems like a natural follow up to improve the PR and make the code readable and less error-prone when compared to the current approach of separate yet logically/semantically related lists.

[RonnyPfannschmidt]

@northernSage as part of the cleanup i recommend turning it into a package and having the pytest_plugin inside the xprocess package as well as _ prefixed modules
this would also prevent issues like #77 by enabling to use find_packages

a quick usage check sould be done to see if any imports happen from 3rd parties

if noboddy else imports this, then its probably fine to just go ahead and use pytest_xproces as root package name to ensure that detail

[northernSage]

I agree. I'll try to get this and other cleanup suggestions ready for our next release 👍🏻

[northernSage]

addressed in #85