Clarify how HLS is supposed to access information about files

[michaelpj]

At the moment it's a little unclear how HLS (rules, plugin handlers, everything really) is supposed to access information about files.

We have two possible sources of information:

• The VFS
• The filesystem

Where do we look? What do we do if something is not in the VFS?

After some discussion on IRC, @fendor and I came to the following conclusion:

• The point of the VFS is to handle files that are managed by the client, and whose content may differ from the filesystem.
• For files that are not in the VFS, and hence not managed by the client, it is totally fine to get them from the filesystem.

See also the prose in the spec: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_didOpen

However, at the moment, we are most often using e.g. getFileContents, which only reads from the VFS. That might be fine (it avoids doing filesystem IO, which might be desirable), but it should be very obvious. There is a function that falls back to the filesystem, the confusingly named getSourceFileSource, but it's not as widely used.

We should clarify the situation and try to make things more consistent.

[michaelpj]

Might also be a good candidate for a hlint warning.