[Backend] Allow chain of ../ at beginning of paths

This MR addresses #9 (closed). We now allow two kinds of prefixes (if any) for paths: / (paths from roots) or an arbitrarily long chain of ../ (paths w.r.t. current directory).

Added malformedImports.dcs to demonstrate some erratic imports and importBadImport.dcs to demonstrate how importing a file that has an import/parse error propagates that error. Renamed a few other test files now that we have more specific tests. E.g., imports.dcs -> importMissingFile.

Removed ./ from imports in all test files.