Fix docs: Move source_url to project

[ftes]

https://hexdocs.pm/ex_doc/Mix.Tasks.Docs.html#module-using-source_url-and-source_ref-together

shows this example

@version "0.30.10"
def project do
  [
    ...
    version: @version,
    docs: docs(),
    ...
  ]
end

def docs do
  ...
  source_ref: "v#{@version}",
  source_url: @source_url,
  ...
end

Which is incorrect afaict. source_url is not a docs option and should be moved to the project function.

[josevalim]

I am looking at the source code and the ones in the top level are simply moved inside:

  defp normalize_source_url(options, config) do
    if source_url = config[:source_url] do
      Keyword.put(options, :source_url, source_url)
    else
      options
    end
  end

  defp normalize_homepage_url(options, config) do
    if homepage_url = config[:homepage_url] do
      Keyword.put(options, :homepage_url, homepage_url)
    else
      options
    end
  end

So I am thinking both are supported and the docs are correct. Thoughts?

[ftes]

I am looking at the source code and the ones in the top level are simply moved inside:

Ah, interesting. I see this behaviour for generating source links:
Top-level project.source_url takes precedence (example).
It overwrites docs.source_url if both are present.
Only if project.source_url is not present, then docs.source_url is used.

Since

1. docs.source_url is not mentioned as a config option elsewhere
2. project.source_url takes precedence

I think the example in question should be changed to use project.source_url.

Going further, maybe docs.source_url should even be deprecated?
I find it unnecessarily complex and thus potentially confusing without further explanation.
Unless there are valid use cases when you don't want project.source_url but do want docs.source_url.

[josevalim]

I believe the original goal was to have source_url/homepage_url also be used by Hex to list in the packages page, but I think it was never implemented.

[josevalim]

All other examples used it at the root, so I merged it for consistency. I don't think it is worth deprecating though as to not push users through this hassle. Thanks!