move glob matching into its own file (#5945)

### Problem

See #5871, where we describe an encapsulation leak created by implementing all of the glob expansion logic in the body of `VFS`.

### Solution

- Create `glob_matching.rs`, exporting the `GlobMatching` trait, which exports the two methods `canonicalize` and `expand`, which call into methods in a private trait `GlobMatchingImplementation`. 

**Note:** `canonicalize` calls `expand`, and vice versa, which is why both methods were moved to `glob_matching.rs`.

### Result

Orthogonal glob matching logic is made into a trait that is implemented for all types implementing `VFS`, removing the encapsulation leak. The `VFS` trait is now just four method signature declarations, making the trait much easier to read and understand.

src/rust/engine/fs/fs_util/src/main.rs

@@ -10,7 +10,7 @@ extern crate protobuf;
 
 use bytes::Bytes;
 use clap::{App, Arg, SubCommand};
-use fs::{ResettablePool, Snapshot, Store, StoreFileByDigest, VFS};
+use fs::{GlobMatching, ResettablePool, Snapshot, Store, StoreFileByDigest, VFS};
 use futures::future::Future;
 use hashing::{Digest, Fingerprint};
 use protobuf::Message;