Make LSP features work even if we can't find a module

[certik]

When opening a file (module or a program) that imports other modules but we can't find their .mod files, here is how it should behave:

• LFortran will still construct ASR on a best-effort basis (by guessing and skipping stuff as needed)
• It will report errors "module X not found" for each use X statement
• VSCode will put red squiggles over each such use X, so the user will know that we can't find the .mod files
• But all LSP features will still work, based on the best-effort ASR that LFortran internally creates; for example VSCode will still show all document symbols