Merge release/dev17.10 to main (#10027)

* Avoid creating ProjectSnapshotManager in CSharpVirtualDocumentFactory

`CSharpVirtualDocumentFactory` unnecessarily forces the
`ProjectSnapshotManager` to be created just to see if it contains any
projects. To avoid this, I've added a `TryGetInstance(...)` method that
returns the `ProjectSnapshotManager` only if it has been created.
Then, in `CSharpVirtualDocumentFactory` it avoids force-creating the
`ProjectSnapshotManager` if `TryGetInstance(...)` returns false.

* Avoid creating ProjectSnapshotManager in FallbackProjectManager

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Co-authored-by: Dustin Campbell <[email protected]>
Co-authored-by: David Wengier <[email protected]>

Author: 3 people