SCons: Replace _find_scu_section_name function with SCons builtins

[dustdfg]

Not only that it is a clean up and ammending of custom workaround but it also transparently allows to process SCons specific paths which contains # in the starts (which in turn in the future should allow me to implement SCU for thirdparty sources which should greatly increase compilation speed)

[Repiteo]

More native SCons implementations are always a plus!

[dustdfg]

I've just found that scu_builders.py have a Ctrl+C, Ctrl+V version of method I am deleting in this PR. I think this should be done in this PR too. I've also noticed tendency not to use SCons related stuff inside builders... And when I tried to add from misc.utility.scons_hints import * to scu_builders.py it didn't work 😅 I delayed that question as much as possible but it is time to ask. Why does builders doesn't use SCons? Or if they use where and how...

[akien-mga]

Why does builders doesn't use SCons?

Usually just because the author of the builder was more familiar with Python than the SCons API. Cleaning things up by making use of SCons features is usually a good change.

[dustdfg]

I've just found that scu_builders.py have a Ctrl+C, Ctrl+V version of method I am deleting in this PR.

Oh. I was a bit wrong. It returns in format dir_dir_dir instead of dir/dir/dir so it is for another PR 🤔 or for this... Anyway this PR again can be considered as ready to be merged 😅

[Repiteo]

Thanks!