Move `eng/run.ps1` to `run.ps1` (and other submodule freedoms)

We made some choices back when we were using a Git fork that we should revisit now that the upstream code is contained in a submodule:

• eng/run.ps1 -> run.ps1
  • More generally, we don't have to be concerned about stuffing all of our code into eng to keep it isolated. We can move out eng/doc to doc (or docs), too.
• Remove _ for eng/_core, eng/_util
  • The Go tests traversed the entire Go repository, and _ was how our code was avoiding being tested as if it was part of the standard library.
  • eng/core.
  • Consider other names that are more descriptive than "core" and "util", and put them in the root of the repo?
    • Merging them into one module might help: https://github.com/microsoft/go/issues/402
• Move eng/artifacts to artifacts. This is easier to navigate to, and it's more familiar to devs who also work with the .NET source code.

Start by reviewing the repository layout around eng/run.ps1, eng/doc, eng/_core, eng/_util, and eng/artifacts, along with the submodule-related context in the issue. Define the target names and module boundaries before making changes; done means the proposed directories are consistently relocated without breaking the repository’s Go or build workflows.