Index `Ecto` schema struct

[scottming]

For example, when we define a module like:

defmodule User do
  use Ecto.Schema

  schema "users" do
    field :name, :string
    field :age, :integer, default: 0
    field :password, :string, redact: true
    has_many :posts, Post
  end
end

and we can't go to the definition when the cursor at %Use|r{} right now, it is pretty annoying.

• https://hexdocs.pm/ecto/Ecto.Schema.html#schema/2
• https://hexdocs.pm/ecto/Ecto.Schema.html#embedded_schema/1
• https://hexdocs.pm/ecto/Ecto.Schema.html#embeds_many/3-inline-embedded-schema
• https://hexdocs.pm/ecto/Ecto.Schema.html#embeds_one/3

[scottming]

Currently, for such structs, we have reverted to using this PR: #707 to retrieve definitions from ElixirSense. This is not a problem easily solved by indexing, and as @zachallaun commented on Discord, we may not want to handle such things specially but instead consider macro expansion.