Refactors to simplify the FS proxy implementation

[rwjblue]

This fixes two major bugs in the prior implementation:

CWD "bleed through"

Prior to these changes 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 [email protected] embroider-build/embroider#766
• Closes Failing test demonstrating CWD "bleed through" #43

Broken "last one wins" semantics

Prior to these changes when using existsSync, lstatSync, or statSync would only work if the first argument to new FSMerger() contained the file in question. Breaking both the fundamental purpose of this library and the "last one wins" semantics.

---

TODO:

• Use typescript to help enforce that all valid operations are accounted for
• Remove defaulting of fullPath = relativePath in handleFSOperation (this will "Fix" the bleed through issue, but causes knock on failures)
• Include previously failing tests from Failing test demonstrating CWD "bleed through" #43