cannot Sass-import a file with an eyeglass dependency

Nobody has claimed this yet.

### Background

Given this tree:
```
eyeglass:modules discovered modules
eyeglass:modules :root(snowcat)
eyeglass:modules ├─┬ @fastly/fui-chrome@5.1.1
eyeglass:modules │ └─┬ @fastly/style-guide@6.0.0-beta.4
eyeglass:modules │ └── @fastly/fui-color@1.1.0
eyeglass:modules ├─┬ @fastly/style-guide@6.0.0-beta.4
eyeglass:modules │ └── @fastly/fui-color@1.1.0
eyeglass:modules └── eyeglass@1.6.0
```
and these files:
```scss
// snowcat/app/styles/app.scss
@import 'fastly/variables/all';
…

// @fastly/style-guide/app/styles/fastly/variables/_all.scss
@import '@fastly/fui-color';
…
```

### Problem

I get

```
eyeglass:import @fastly/fui-color cannot be imported from /Users/jamesrosen/Code/snowcat/tmp/eyeglass_compiler-input_base_path-WdAFZ1zV.tmp/fastly/variables/_all.scss +0ms
eyeglass:import @fastly/fui-color cannot be imported from /Users/jamesrosen/Code/snowcat/node_modules/@fastly/style-guide/app/styles/fastly/variables/_all.scss +0ms
```

I've added some logging to [`canAccessFrom`](https://github.com/sass-eyeglass/eyeglass/blob/ea89b37403d9f547cbc09988e968032efe46be3b/lib/modules/EyeglassModules.js#L263-L290) and got
```
// 1
{
branches: [
{
name: 'snowcat',
version: '0.0.0',
path: '/Users/jamesrosen/Code/snowcat',
dependencies: {
'@fastly/fui-chrome': [Object],
'@fastly/style-guide': [Object],
eyeglass: [Object]
}
}
],
name: '@fastly/fui-color',
pkg: '/Users/jamesrosen/Code/snowcat',
origin: '/Users/jamesrosen/Code/snowcat/tmp/eyeglass_compiler-input_base_path-XxBdGBtB.tmp/fastly/variables/_all.scss'
}

// 2
{
branches: [],
name: '@fastly/fui-color',
pkg: '/Users/jamesrosen/Code/snowcat/node_modules/@fastly/style-guide',
origin: '/Users/jamesrosen/Code/snowcat/node_modules/@fastly/style-guide/app/styles/fastly/variables/_all.scss'
}
```

The first one makes sense to me. `@fastly/fui-color` isn't a direct dependency of `snowcat`, so it wouldn't appear in the ("immediate") dependencies.

But the second seems wrong. Why does `@fastly/style-guide` have no `branches`, especially given the `eyeglass:modules` step found the tree?

Start with lib/modules/EyeglassModules.js, especially canAccessFrom at lines 263-290, and trace how the eyeglass:modules dependency tree becomes branches for each package origin. Reproduce the nested @fastly/style-guide and @fastly/fui-color case from the issue, then determine why the import is rejected and verify that the Sass import succeeds with appropriate regression coverage.

Written by the indexing model from the issue text.