The stdliblist operation that the gopackagesdriver relies on fails on
Go 1.18rc1 with the following error:

```
external/go_sdk/src/crypto/elliptic/p256_asm.go:24:12: pattern p256_asm_table.bin: cannot embed irregular file p256_asm_table.bin
```

We see this failure because Bazel builds a symlink tree of the GOROOT run
`go list` with. However, since [CL 380475][1], the Go standard library uses
`go:embed` to embed a file in `crypto/elliptic`, but `go:embed` does not
support symlinks.

[1]: https://go.dev/cl/380475

Fix this by having stdliblist copy the relevant portions of the GOROOT to
run `go list` with. This matches [what the stdlib action does][2].

[2]: https://github.com/bazelbuild/rules_go/blob/e9a7054ff11a520e3b8aceb76a3ba44bb8da4c94/go/tools/builders/stdlib.go#L54-L57

Resolves bazelbuild#3080

Add a dependency on `GoStdLib._list_json` to the stdlib test.
This ensures that we can successfully build the JSON data needed by the
gopackagesdriver.

The prior version of this fix was incomplete because it generated
incorrect relative paths.

For example, before a path would be:

    __BAZEL_OUTPUT_BASE__/external/go_sdk/src/archive/tar/common.go

But with the prior version of this change.

    __BAZEL_OUTPUT_BASE__/src/archive/tar/common.go

It would drop the external/go_sdk from the path because the flattening
logic in stdliblist makes these relative to the execRoot.

We cannot overwrite external/go_sdk in execRoot because that's a path
controlled by Bazel.

Instead, create a parallel external/go_sdk under a new directory "root",
and flatten paths relative to that.

recover tests/core/stdlib/stdlib_files.bzl