Backport SourcePath from the lazy-trees branch

This introduces the SourcePath type from lazy-trees as an abstraction
for accessing files from inputs that may not be materialized in the
real filesystem (e.g. Git repositories). Currently, however, it's just
a wrapper around CanonPath, so it shouldn't change any behaviour. (On
lazy-trees, SourcePath is a <InputAccessor, CanonPath> tuple.)

src/libexpr/tests/libexpr.hh

@@ -28,7 +28,7 @@ namespace nix {
             }
             Value eval(std::string input, bool forceValue = true) {
                 Value v;
-                Expr * e = state.parseExprFromString(input, "");
+                Expr * e = state.parseExprFromString(input, state.rootPath(CanonPath::root));
                 assert(e);
                 state.eval(e, v);
                 if (forceValue)