Support downloading package list behind a proxy

[hfhbd]

Is your feature request related to a problem? Please describe
I want to use dokka behind a proxy and create links to other docs, like stdlib. Currently, I get the error message:
Failed to download package-list from ..., this might suggest that the remote resource is not available, .... This message is technically correct.

Describe the solution you'd like
Best solution: Use native Gradle dependency resolution to fetch the package list file, because you already need to add the proxy settings to Gradle. This would also allow you to cache the files by default and fetch the files only once.

Describe alternatives you've considered
Keep the current code and add a proxy option. This option also downloads the file for each source set again without a cache.

Additional context

Are you willing to provide a PR?
With a start point, yes.

[aSemy]

hey 👋

The package list is downloaded using java.net.URL

dokka/plugins/base/src/main/kotlin/resolvers/shared/PackageList.kt

Lines 26 to 34 in d680b14

	fun load(url: URL, jdkVersion: Int, offlineMode: Boolean = false): PackageList? {
	if (offlineMode && url.protocol.toLowerCase() != "file")
	return null
	val packageListStream = runCatching { url.readContent() }.onFailure {
	println("Failed to download package-list from $url, this might suggest that remote resource is not available," +
	" module is empty or dokka output got corrupted")
	return null
	}.getOrThrow()

Have you tried setting -Djava.net.useSystemProxies=true, like described in this answer https://stackoverflow.com/a/18164472/4161471?

Alternatively you could use Gradle to download the package list file and pass it to packageListUrl

dokka/runners/gradle-plugin/src/main/kotlin/org/jetbrains/dokka/gradle/GradleExternalDocumentationLinkBuilder.kt

Lines 58 to 70 in d680b14

	/**
	* Specifies the exact location of a `package-list` instead of relying on Dokka
	* automatically resolving it. Can also be a locally cached file to avoid network calls.
	*
	* Example:
	*
	* ```kotlin
	* rootProject.projectDir.resolve("serialization.package.list").toURL()
	* ```
	*/
	@Optional
	@Input
	val packageListUrl: Property<URL> = project.objects.property()

The use of URL is related to #2993

[IgnatBeresnev]

Related issue: #2376

[hfhbd]

Another idea: Publish the package list as gradle artifact, or include it in the sources jar.