FixLoads issue / can we have forced loads?

[tzaman-stripe]

This test case in fix_test.go fails:

"this test fails": {
    input: `foo_binary(
    name = "a",
    field = [magic_macro],
)
`,
    want: `load("@bar", "magic_macro")
load("@foo", "foo_binary")

foo_binary(
    name = "a",
    field = [magic_macro],
)
`,

I can help PR a fix, but would it be possible to have generate force certain rules to be loaded? The current FixLoads code seems fairly brittle, and it'd be nice to add in certain loads no matter what.

[fmeum]

I would also like to see improvements to FixLoads, specifically to force loads to use a particular .bzl file, updating the label even if some load already exists for the symbol.