Watch changes on directories

[msujew]

Closes #1230

Even though I expected this to be a small fix, this was surprisingly complicated. The main issue is that we don't get notified what is contained in the directory that has changed. Therefore, we need to "manually" dig through the changed directory. Performs a few changes:

1. Adds a UriTrie data structure. It's a tree structure for document storage that respects directories (compared to the previous flat map). It is required to quickly figure out which documents belong to a changed/deleted directory.
2. Adjusts the document builder to find all documents that belong either to a changed directory or a deleted directory.
3. Adds a few new methods to the workspace manager so we can efficiently traverse newly added folders.
4. Adds a new testing utility VirtualFileSystemProvider that can be programmed for specific tests.

The behavior is tested by some integration tests, and can be manually tested by deleting/renaming directories with DSL files within them. The old documents should be correctly deleted and the documents from the renamed directory should've been correctly added to the documents service.