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/libutil/canon-path.hh

@@ -46,6 +46,8 @@ public:
         : path(std::move(path))
     { }
 
+    static CanonPath fromCwd(std::string_view path = ".");
+
     static CanonPath root;
 
     /**