Fix eager loading of HasManyThrough associations

[groue]

This pull request fixes some bugs with eager loading of HasManyThrough associations.

The bugs may occur when two conditions are met:

• One of the intermediate steps of the HasManyThrough association is a "to-one" association (belongsTo, hasOne, hasOneThrough).
• The HasManyThrough association is eager-loaded in a request that also involves that to-one association.

For example, given this schema:

1. A player belongs to a team
2. A team has many competitions
3. A player has many competitions through its team.

extension Player {
    static let team = belongsTo(Team.self)
    static let competitions = hasMany(
        Competition.self,
        through: team,
        using: Team.competitions)
}

Using both Player.team and Player.competitions is a single request may lead to unexpected or inconsistent behaviors.

A sample request which was not behaving correctly:

// WRONG
let buggyRequest = Player
    .including(required: Player.team)
    .including(all: Player.competitions)

After this PR is merged, buggyRequest will fatalError, complaining that the association key "team" is used in incompatible ways. Future GRDB versions may handle it, but GRDB 4.0 does not.

One possible fix is to include the competitions from the team:

// CORRECT
let fixedRequest = Player
    .including(required: Player.team
        .including(all: Team.competitions))

Another way to mix Player.team and Player.competitions in a single request is to make sure both teams (the one from Player.team, and the one hidden inside Player.competitions) use different association keys. For example:

// CORRECT
let request = Player
    .including(required: Player.team
        .filter(Team.Columns.isSelected == true)
        .forKey("selectedTeam"))         // "selectedTeam"
    .including(all: Player.competitions) // "team"