Caching of type-safe model accessors didn't take differences in container elements into account (#1303, #1304)

In order to speed up the compilation of Kotlin DSL build scripts and amortise the cost of the required code generation step, the Kotlin DSL script compiler tries to maximise the reuse of generated type-safe model accessors across projects and builds.

It does that by caching the generated code based on a key defined by, among other things, the set of accessible Gradle model elements in the target project.

In previous versions however, the cache key definition was missing elements from Gradle model containers such as tasks and sourceSets which would cause accessors generated for one project to be (wrongly) reused for another project when they differed only in the set of accessible container elements.

This defect has been fixed.