This is a necessary preparation for moving the C# dependency management to `paket`,
which in turn is a necessary preparation for moving the C# build to bazel.
As we discovered in https://github.com/github/codeql/pull/16376,
`paket` tries to restore all projects recursively from the root folder.
If we support building C# code under both `ql/csharp` and `ql/cpp`, we need
to have a single lockfile under `ql`, as both codebases share the same set of dependencies
(and utilities from `ql/csharp/extractor`).
Then, `paket` will also try to restore things that look like "C# projects" in other languages'
folders, which is not what we want.
Therefore, we address this by moving all C# code into a common root directory, `ql/csharp`.
This needs an internal PR to adjust the buildsystem to look for the autobuilder in the new location.
