Make path values lazy

This fixes the double copy problem and improves performance
for expressions that don't force the whole source to be added to the
store.

Rules for fast expressions:

- Use path literals where possible
   - import ./foo.nix
- Use + operator with slash in string
   - src = fetchTree foo + "/src";
- Use source filtering, lib.fileset

- AVOID toString
- If possible, AVOID interpolations ("${./.}")
- If possible, move slashes into the interpolation to add less to the store
   - "${./src}/foo" -> "${./src/foo}"

toString may be improved later as part of lazy-trees, so these
recommendations are a snapshot. Path values are quite nice though.

src/libutil/source-accessor.cc

@@ -105,4 +105,8 @@ CanonPath SourceAccessor::resolveSymlinks(
     return res;
 }
 
+bool SourceAccessor::toStringReturnsStorePath() const {
+    return true;
+}
+
 }