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/paths.cc

@@ -0,0 +1,10 @@
+#include "eval.hh"
+
+namespace nix {
+
+SourcePath EvalState::rootPath(CanonPath path)
+{
+    return std::move(path);
+}
+
+}