Allow using version catalog plugins without version

[asodja]

Fixes #18236
Fixes #18467

Changes:

• plugins can now have no version in version catalog: with API this can be done with alias("alias").toPluginId("id").withoutVersion(), in TOML you can do any of:

[plugins]
my-plugin1 = "plugin.id"
my-plugin2 = { id = "plugin.id" }
my-plugin3 = { id = "plugin.id"  version = ""}

• generated accessors for plugins now implements PluginDependencyProvider that have getWithoutVersion() method, so you can do:

plugins {
    alias(libs.my.plugin.withoutVersion)
}

• PluginDependenciesSpec now implements alias as default method due to 7.2: PluginDependenciesSpec interface adds non-default methods, thus breaking backward compatibility #18467

---

I am not perfectly happy with alias(libs.my.plugin.withoutVersion) it feels verbose, but other solutions are trickier.

Maybe alternative alias(libs.my.plugin).withoutVersion() could be interesting, but in that case we should change logic of alias(libs.my.plugin).version(null): now it fails, but it could set version to unspecified. Interestingly docs of PluginDependencySpec already say that null does that.

[github-actions]

❌ Contributor CI Build failed: https://ge.gradle.org/s/xdynfibit7uiq

[github-actions]

❌ Contributor CI Build failed: https://ge.gradle.org/s/7zwsumr6735y4

[asodja]

@bot-gradle test QFL

[bot-gradle]

OK, I've already triggered QuickFeedbackLinux build for you.

[asodja]

@bot-gradle test this

[bot-gradle]

OK, I've already triggered ReadyForMerge build for you.

[asodja]

I probably should also restrict withoutVersion keyword in plugin... Yeah, this is one more thing why I am not that happy with alias(libs.my.plugin.withoutVersion)

[ljacomet]

Closed in favor of #19015