Bundled Kotlin stdlib preferred over classpath dependencies #15020

Caused by: java.lang.NoSuchMethodError: kotlin.collections.ArraysKt.copyInto([B[BIII)[B
        at okio.Segment.writeTo(Segment.kt:169)
        at okio.Segment.compact(Segment.kt:152)
        at okio.Buffer.write(Buffer.kt:1842)
        at okio.Buffer.read(Buffer.kt:1854)
        at okio.Buffer.writeAll(Buffer.kt:1642)
        at okio.Options$Companion.buildTrieRecursive(Options.kt:187)
        at okio.Options$Companion.buildTrieRecursive(Options.kt:174)
        at okio.Options$Companion.buildTrieRecursive$default(Options.kt:113)
        at okio.Options$Companion.of(Options.kt:72)
        at okhttp3.internal.Util.<clinit>(Util.kt:71)
        at okhttp3.internal.concurrent.TaskRunner.<clinit>(TaskRunner.kt:309)
        at okhttp3.ConnectionPool.<init>(ConnectionPool.kt:41)
        at okhttp3.ConnectionPool.<init>(ConnectionPool.kt:47)
        at okhttp3.OkHttpClient$Builder.<init>(OkHttpClient.kt:471)
That means when developing plugins in Kotlin we have to be very aware of not exceeding stdlib compatibility for the bundled Gradle Kotlin standard library version.
Expected Behavior
The buildscript classpath should win over any default dependencies.
Current Behavior
Default dependencies win over buildscript classpath dependencies.
Context
It makes maintaining plugins difficult and makes adoption of later Kotlin features difficult, causing us to resort to shading to provide classpath isolation, which comes with its own problems.
Steps to Reproduce
Take the following build:
buildscript {
    repositories {
        mavenCentral()
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-stdlib:1.4.10"
// Kotlin 1.x class
println Class.forName("kotlin.SynchronizedLazyImpl").package.implementationVersion
// Kotlin 1.4-only class
println Class.forName("kotlin.properties.PropertyDelegateProvider").package.implementationVersion
Run ./gradlew help and note the output:
1.3.72-release-468 (1.3.72)
1.4.10-release-411 (1.4.10)
Your Environment
Gradle 6.7
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