[ENH] make get_packages_with_changed_specs safe to mutation of return

[fkiraly]

If an lru_cache-d function returns a mutable object, and that object is later muted, the cached function wlil continue returning the muted - incorrect - return.

This PR fixes the one instance in the code base where this issue could arise.

[yarnabrina]

(Not a review, because I've very limited experience with lru_cache method.)

Can you please explain what is the current issue, and in which scenario it has an adverse effect?

[fkiraly]

Sure. Let's suppose we have the following method that returns lists of length n (but imagine it is a much more tedious, resource consuming operation that happens internally).

def compute_sequence(n):
    return list(range(n))   # imagine the logic is more resource intensive and tedious

If we now lru_cache the method compute_sequence, and do the following:

my_sequence = compute_sequence(42)
my_sequence[3] = 4242

then every subsequent call compute_sequence(42) will return the changed list, until it is mutated again.

This is admittedly a low risk for the function in this PR, given that it is used only internally, but from the above I derive that no lru_cache-d method should have a mutable return.

[yarnabrina]

Understood, thanks for the explanation.