Failing test demonstrating CWD "bleed through"

[rwjblue]

The various FS APIs that are wrapped do not properly scope requests to the input source directories.

For example, given this directory structure:

├── bar
│   └── baz
├── foo
└── qux

If the current working directory is the directory above, the following returns the incorrect result:

let merger = new FSMerge(['bar', 'qux');
merger.fs.existsSync('foo');

// -> currently returns `true`, but should obviously be `false

If ran from a working directory that did not have a foo/ folder, it would return the correct result.

References:

• Ensure current working directory files do not affect broccoli-funnel operation broccoli-funnel#136
• [@embroider/util] Invalid dependency on broccoli-funnel embroider-build/embroider#764
• Update to broccoli-funnel@3.0.5 embroider-build/embroider#766

[rwjblue]

(made from the wrong branch 😭, replaced by follow up PR)