Allow Parts to Self-Ref Attributes

Suppose I have something like:

[FreeTDS_installation]
recipe = zc.recipe.cmmi
url = blah
odbcinst_ini =
    [FreeTDS]
    Driver = ${FreeTDS_installation:location}/lib/libtdsodbc.so

The initial case above fails to parse because at the time the ${} expression is reached during read-in time, the part section has not yet been added to the global registry of part sections. Hence I get a "FreeTDS_installation" undefined error.

It just seems there ought to be a way for a part section to refer to its own location attribute.