The types in adl stdlib are implicitly available to adlc.
By adding the output from adlc show --adlstdlib to adl.packageRoots these are available to the lsp.
IMO adl stdlib should be implicitly add to the end of adl.packageRoots
In the rare case of hacking on the stdlib its self, a custom path could override the implicitly added one.
Finding the location might be difficulty without assume adlc is already installed and in the path.
(but is adlc isn't installed then the adl stdlib files would be on the file system)