Proposal to remove pystring dependency

[KevinJW]

Our current usage of pystring falls into 2 camps

String manipulation e.g. split, startswith, endswith, splitlines, lstrip etc
Filesystem interaction pystring::os::*

I would propose that this be carried out in 3 stages:

1. Replace string functionality with code based on our own StringUtils (add functionality where needed). We already have some tests for replacing the split() behaviour for example.
2. Replace filesystem interaction by calls to std::filesystem (C++17)
3. Remove all pystring from CMake

[meimchu]

Based on some work I did in #2310 I think there is value in adding some filesystem functionality to PathUtils and am interested in exploring this further if still desired.

Some of the name may need to be different in order to avoid confusion. FileExists() is one where its current use case seems quite specific with caching hashes. If we want some basic filesystem usages such as filesystem::exists, it will probably need a different name than FileExists() to avoid confusion?

I can take a look at the first stage and replace pystring::split() with existing StringUtils::Split() and go from there.

[meimchu]

I have created a PR that should address the first phase and would love to get some feedback on it. Thank you! #2313