Parallel assignment of local variables in packages can fail

[d-torrance]

I just discovered a bug with the new parallel assignment feature. If we try to assign variables alongside other things (i.e., not everything is a local variable) using parallel assignment inside a package, then we get a "mutable unexported symbol" error.

For example, in the package Foo, if we have:

f = () -> (
    (,x) := (1, 2);
    x)

then we get:

i1 : loadPackage "Foo"
stdio:1:11:(3):[1]: error: mutable unexported unset symbol(s) in package Foo: 'x'
Foo.m2:4:6-4:7: here is the first use of 'x'