Fix broadcast_indices

[nalimilan]

This fixes a regression introduced in 4f1b479. broadcast_indices() needs to
be overloaded by packages for custom types, so it cannot be hidden under
_broadcast_indices(). Also, ::Type is incorrect since the method only applies
to scalars.

Make the tests more complex to be closer to actual implementations in packages
so that regressions like this will be noticed in the future.

Fixes a bug in DataArrays: JuliaStats/DataArrays.jl#259 (comment)

[nalimilan]

I just realized we already have _containertype which starts with an underscore and yet needs to be overridden by packages. It's really weird that non-exported, underscored functions would be part of the public API... Cf. design discussion at #20740.

[andyferris]

Yes, underscored and not exported usually means the exact opposite of public API.

[KristofferC]

Good to go?

[Sacha0]

lgtm! :)

[nalimilan]

Thanks. It would still be interesting to have @vtjnash double-check this, in case we missed something.

[KristofferC]

Sorry, I missed the review request.

[nalimilan]

No worries, four people considered it was a reasonable change, it passed the tests, and it fixes DataArrays tests. So the burden of the proof is on Jameson! ;-)

[vtjnash]

it passed the tests, and it fixes DataArrays tests

you might need better tests... :P (yes, I realize this is also my fault for not adding a test when I fixed this code)

This PR broke cases like the following:

convert.(Nullable, [1, 2])
convert.(Any, [1, 2])

(causes them to compute the wrong size, shape, and type)