Fix apparent corner case with import/no-commonjs rule

[charlessuh]

var x = require("x") will throw an error, but var x; x = require("x") does not.

[coveralls]

Coverage increased (+2.5%) to 97.937% when pulling 5d05134 on charlessuh:assign-no-commonjs into f63dd26 on benmosher:master.

[coveralls]

Coverage increased (+2.5%) to 97.937% when pulling 5d05134 on charlessuh:assign-no-commonjs into f63dd26 on benmosher:master.

[coveralls]

Coverage increased (+2.5%) to 97.937% when pulling 5d05134 on charlessuh:assign-no-commonjs into f63dd26 on benmosher:master.

[coveralls]

Coverage increased (+2.5%) to 97.937% when pulling 1edbbd0 on charlessuh:assign-no-commonjs into f63dd26 on benmosher:master.

[ljharb]

I think this can be called a bugfix, even though it will cause more warnings to be issued.