Request referenced files from the client instead of attempting to load from the language service

Currently any files that are referenced from an edited script (i.e. dot-source references) are being resolved and loaded by the language service. Since we want to target remote editing scenarios (language service running in a web service), we need to shift the file resolution responsibility to the client so that it can resolve files with respect to its own workspace.

This will require the introduction of new protocol request/response types which allow the server to request files from the client. The core PSES API will handle this behavior through an interface so that file resolver implementations can be chosen at runtime.

This bug in vscode-powershell should benefit from the improvement: https://github.com/PowerShell/vscode-powershell/issues/31

Start by tracing how the language service currently resolves dot-source references and review the proposed protocol request/response types and core PSES file-resolver interface. Done means the client resolves referenced files in its own workspace, the server requests them through the protocol, and resolver implementations can be selected at runtime.