non-`file:` URIs in library/userThirdParty settings don't work

my extension generates a LuaLS userThirdParty bundle dynamically from Factorio's doc files (from the local game install, generally). I'm working on trying to switch to using a virtual FS provider for these files instead of cluttering up real workspace-associated folders with them. I've registered as the provider for fmtk: uris, and i can open all the files for the generated bundle in vscode in editor tabs, but LuaLS won't load them as libraries.

I set

"Lua.workspace.userThirdParty": [
        "fmtk:/luals"
    ],

in my settings.json, instead of setting the generated folder path as i had done previously, but LuaLS seems to have completely ignored it.

I also tried

    "Lua.workspace.library": [
        "fmtk:/luals/factorio/library"
    ],

but that also seems to ignore anything that isn't a plain file path.

Start by reproducing the issue with the fmtk: virtual file-system provider and the Lua.workspace.userThirdParty and Lua.workspace.library settings shown in the report. Trace how these settings resolve library paths; done means non-file URIs are loaded as libraries rather than ignored, with a regression test covering the behavior.