Prepare deprecation of full index and action usage

[mbargull]

Description

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[mbargull]

@kenodegard, @jezdez, I noticed we need to deprecate some more things when we replace the conda.core.get_index and install_actions usage.
This is based on gh-5151 and gh-5074, hence the large overall changeset.
The actual changes are only in commit 7ea64c6 .

What this does overall is:

1. Mark channel data functionality in conda_build.index (now in conda-index) as deprecated since it requires iterating over the full index.
2. Mark actions-related arguments as deprecated since actions is now always empty or a {PREFIX_ACTION: str, LINK_ACTION: list[PackageRecord]} dictionary for which the prefix is redundant because the call sites always have the prefix already which in turn means actions is really now equivalent to a list of PackageRecord.
   We want to remove the install_actions functionality which manually overrides the solver's index with the full index.
   As such, this PR prepares the functions and their arguments to just handle lists of PackageRecord so we can do away with all the legacy actions stuff in the next version.

[mbargull]

I marked this explicitly as deprecated since I know there are some conda-forge scripts that use _apply_instructions.
(We probably shouldn't to continue to use the underscore-prefixed function from conda-idex either. But this way we can at least mark it for removal here.)

I assume we don't have to mark every single underscore-prefixed symbol here separately, right?!

[jezdez]

Every function that we know is used and plan to remove, independent whether it's prefixed with an underscore or not, is going to need the decorator.

Remember Hyrum's Law, everything is API in the eye of the user, we can only manage the way it's being used and informing the users about our intentions, is good engineering practice.

[mbargull]

Every function that we know is used [...]

The problem is, we of course don't know which are used in the wild...
I've now just plonked the decorator on everything in conda_build.index 🤷.

(If anyone knows a convenient way to really handle API<->internals definition in Python which we could use for future modules/packages, I'm all ears. Python, since it's one of those "everything is public and dynamic" languages, is just awful when Mr. Wright et al. point out these issues...)

[dholth]

Nice, thanks for working on the index feature.

Optional channeldata in standalone conda-index https://github.com/conda/conda-index/blob/main/conda_index/cli/__init__.py#L134-L141

[jezdez]

Every function that we know is used and plan to remove, independent whether it's prefixed with an underscore or not, is going to need the decorator.

Remember Hyrum's Law, everything is API in the eye of the user, we can only manage the way it's being used and informing the users about our intentions, is good engineering practice.

[mbargull]

Needs news/CHANGELOG.md entries.
I'm afk for a bit, so feel free to add things at will.

[mbargull]

Thanks for the updates and reviews!
I'm out for today -- GL HF on releasing! 🤞