We currently require that server paths are full paths, which is
fairly inconvenient for a myriad of reasons. This commit
attempts to resolve a given server path with the current workspace.
This has a nice additional affect that we can now actually have
default server paths. This means that mlir-lsp-server and
mlir-pdll-lsp-server can be transparently picked up from
build directories (i.e. generally no need for upstream users to
configure the extension).
Fixes #54627
Path feels weird here as these seem like tool names instead while the actual path is computed in caller.