Gets a data-flow node where this value enters the current codebase.
For example:
# API::moduleImport("re").asSource()
import re
# API::moduleImport("re").getMember("escape").asSource()
re.escape
# API::moduleImport("re").getMember("escape").getReturn().asSource()
re.escape()
This predicate does not include nodes transitively reachable by data flow;
use getAValueReachableFromSource
for that.