Ensure compression parent is also visible

[colin-grant-work]

What it does

Fixes a bug in which folders could get compressed into an invisible workspace root.

How to test

1. Create a directory that has a single, empty directory as its only child.
2. Open that directory as a workspace.
3. Observe that the child folder is visible under both settings of the explorer.compactFolders preference.
4. Use the examples/compression-example workspace to ensure that other compression features continue to work as usual.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes look good to me 👍
I confirmed that it fixes the bug, and that the compression-example works well.

master:

pull-request:

The workspace structure looked like:

$ tree
.
|____.theia