Merge "amo" and "core" packages

[willdurand]

This is only be possible after #13735. We only have one app in this project so let's remove the amo and core folders and merge some of the common files together.

For QA: this is a major code change but it shouldn't have any impact on the site.

[lakshyabatman]

Hello, I'm interested to work on this issue as well. Please let me know if it's possible. Thankyou.

[willdurand]

No, it's not, sorry.

[lakshyabatman]

No problem, thankyou.

[willdurand]

You can find issues by looking at the opened contrib: welcome issues.

[bobsilverberg]

@willdurand After mozilla/addons-frontend#9872 lands, we'll be one step closer to being able to do this. I was thinking we might want to continue taking small steps, as opposed to just trying to merge all of the code in one big patch. For example, maybe we can start by moving individual things out of core and into amo? What do you think of taking that approach?

Also, do you imagine that everything will end up in amo, or do you envision doing away with both amo and core and just having everything live under src?

[willdurand]

I think we can get rid of both amo and core. We can land incremental patches.

[willdurand]

Here is the plan:

1. move core sources to amo (we need a directory level in src/ otherwise it breaks our absolute imports)
2. move core test files to amo
3. move core stories to amo

It will require 2 or 3 PRs I think.

I'll file an issue for the ui package (edit: see #13927)

[ioanarusiczki]

@willdurand Looks good. Release testing is completed - on AMO stage with FF84(Win).