Give up on package uploads

One way to resolve issues similar to #392, #382 and #335 once and for all is to give up on the ability to upload packages directly and just reuse package-sets as a source for docs generation: it is already a requirement for all packages in package-sets to be published on bower.

Moreover, doing so will make impossible the situation when some package is not found in the bower registry, but its docs are still on pursuit (obviously, this will be true up to delays between SearchIndex updates).

Currently there are three "registries": bower, pursuit and package-sets, and the latter two are only partially intersecting. This situation is worse than having only two, with one containing a subset of packages of the another.

The proposed change will make it harder to publish documentation on pursuit, as it will require making a PR to package-sets, but OTOH we'll be able to guarantee that if something is found on pursuit, it will just work, which would be beneficial for all, especially for new users of the language.

Additionally, the change will open a straightforward way to implement #267 .

Start by tracing the package-upload and documentation-generation entry points, then inspect how package-sets, Bower, and SearchIndex interact. Done means documentation generation uses package-sets as its source, direct package uploads are removed, and packages found on Pursuit are available through the Bower-backed flow.