Python: Make "Modification of parameter with default" flow-sensitive.

[taus-semmle]

Extends the analysis in py/modification-of-default-value to use taint tracking. This enables us to detect modifications that happen inside other function calls.

@felicity-semmle for the change note. (And also possibly the alert message?)

Open question:

I have encountered the following idiom in some places that are flagged by the current query

def foo(x=[]):
    if not x:
        x = [1]
    # ... more statements, none of which modify x

Should we treat if not x as a sanitiser for x? As far as I can tell, there are no hidden surprises in the above code.

[felicitymay]

Thanks for pinging me. One minor suggestion, otherwise the text lgtm.

[ghost]

All committers have signed the CLA.

[felicitymay]

Thanks for the change. Text LGTM.

[markshannon]

LGTM. We can add handling of taint truthiness for another PR.