Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

Migrating `Common::CaseInsensitiveLess` to StringUtil.h will hopefully discourage rolling one's own solution in the future for case-insensitive associative containers when this (quite robust!) solution already exists.

`Common::CaseInsensitiveStringCompare::IsEqual` was removed in favor of using the `Common::CaseInsensitiveEquals` function.

The `a.size() != b.size()` condition in `Common::CaseInsensitiveEquals` can be removed, since `std::ranges::equal` already checks this condition (confirmed in libc++).

Gets this out of the global namespace and into the Common namespace

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Now that the std::map less-than comparitor is capable of being used with
heterogenous lookup, we're able to convert many of the querying
functions that took std::string references over to std::string_view.

Now these functions may be used without potentially allocating a
std::string instance unnecessarily.