Fix accessing imported modules’ globals in list comprehensions

[minitech]

What I hope is a correct fix for a bug I encountered with this code:

import sys

bwt = [
    "" if c == "$" else c
    for c in sys.argv[1]
]

burrow.codon:42:14-25: error: name 'argv' is not defined

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[minitech]

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[inumanag]

Thank you for this! I have merged it to #515.