AddImportsVisitor may add imports after usage

[andrecsilva]

Consider the following, suppose the c() call was just added by another transformation/codemod:

c()
import a
from b import d

Trying to add a c import will result in it being added after the call:

In [1]: import libcst as cst

In [2]: from libcst.codemod.visitors import AddImportsVisitor

In [3]: from libcst.codemod.visitors._imports import ImportItem

In [4]: print(cst.parse_module("c()\nimport a\nfrom b import d").visit(AddImportsVisitor(cst.codemod.CodemodContext(), [ImportItem('c',None,None)])).code)
c()
import a
from b import d
import c

In [5]: print(cst.parse_module("c()\nimport a\nfrom b import d").visit(AddImportsVisitor(cst.codemod.CodemodContext(), [ImportItem('b','c',None)])).code)
c()
import a
from b import c, d

A possible fix for that is to change it so that imports are only added at the top of the file.