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/libcmd/editor-for.hh

@@ -2,11 +2,12 @@
 ///@file
 
 #include "types.hh"
+#include "input-accessor.hh"
 
 namespace nix {
 
 /* Helper function to generate args that invoke $EDITOR on
    filename:lineno. */
-Strings editorFor(const Path & file, uint32_t line);
+Strings editorFor(const SourcePath & file, uint32_t line);
 
 }