Invalid path `....` for file relative import path. Should be a forward relative path.

Hey there! I found an error message that's a bit confusing to me, and so I thought you all might appreciate that feedback.

I am working on depending on a Rust project that has a simple buildscript. I am using reindeer to make this work.

I can elaborate on all of the details, but most of them don't matter. Point is, I have this:

`toolchains\buildscript.bzl`: defines `buildscript_args`

`third-party\BUCK`: first line is `load("//toolchains/buildscript.bzl", "buildscript_args")`

I figured `//toolchains/buildscript.bzl` would be the proper target to include this.

When I build, I get this error:

```text
6: The `load` at third-party/BUCK:1:6-36 of `root//toolchains/buildscript.bzl` should use the canonical name `toolchains//buildscript.bzl`
```

Okay, sure. If I change the `load` to use that name instead. That gives me this:

```text
6: Invalid path `toolchains//buildscript.bzl` for file relative import path. Should be a forward relative path.
```

I tried poking around, but I'm not sure what a "forward relative path" is. A pointer towards what that is would be helpful. I'm not sure if this truly is a "this error message could be improved" situation or a "steve just needs to learn more terms" situation, but I figured that when the previous error suggested something that leads to another error, this may be a place where things could be smoothed out.