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.

Continues the migration of our code over to the fmt logger.

These strings are only used for comparison against other strings, so a
string view can be used here.

Otherwise DolphinQt will have a stale symbol list and
you can get nullptr dereferences when trying to use it.

Moves more common code into the Common namespace where it belongs.

Given this is a class with non-trivial data-members, it's preferable to
default the constructor and destructor in the cpp file.

Normalizes variable naming so that it adheres to our coding style

While we're at it do minor cleanup relating to modified lines

Given a std::map can't have duplicate keys, iterating over the map
explicitly isn't necessary, and find() can just be used instead.

Also, instead of manually calling push_back() for every entry to
be added, the range constructor of std::vector can be used instead to add
the whole range all at once.