kotlin

Provide correct classpath for script plugins with buildscript blocks

- Make `ScriptSource` resource descriptions the same as core Gradle

- Make child `ClassLoaderScope` ids similar to core Gradle but avoid clashes

The body of script plugins will still be executed when they are

applied from other scripts as they might affect the classpath of the

applying script.

- It is faster than going through the `ScriptPluginFactory`

indirection

- We can more easily change its API

by moving them to the :provider-plugins project

and adding 2 SPI interfaces in order to share code with :provider

Signed-off-by: Paul Merlin <paul@gradle.com>

The classpath of a precompiled script plugin is the compile classpath

of its enclosing source-set.

See #670

in order not to impact all other build executions

by extracting classpath mode switch to its own type

A BuildSrcProjectConfiguration action creates a task that generates a

text file containing the source roots for the root buildSrc project and

all its runtime project dependencies.

This allows for correct buildSrc source roots resolution when editing

kotlin settings or build scripts in the presence of a multi-project

buildSrc build.