[TypeRecovery] include <returnValue> dummy for index accesses of calls

[xavierpinho]

It was observed (#4558) a discrepancy between typeFullNames in Python for the following two samples:

from helpers import foo
x = foo()
y = x[0] # y.typeFullName = helpers.py:<module>.foo.<returnValue>.<indexAccess>

vs

from helpers import foo
y = foo()[0] # y.typeFullName = helpers.py:<module>.foo.<indexAccess>

The difference is the missing <returnValue> dummy in the second sample. This patch checks (in getIndexAccessTypes) whether the indexAccess' target is a call and that there's already in the symbol table an entry for it.