Add core-lambda-stubs to classpath

Add core-lambda-stubs to the bootclasspath for modules with no
sdk_version or with sdk_version: "current", and to the classpath
for modules that specify a specific sdk_version number.  Use the
prebuilt sdk-core-lambda-stubs when using a prebuilt SDK.

Fixes compiling modules with lambdas against the SDK.

This reapplies I5e700f2dd86f1a6b84b7a55dd9bfe21a448d3fb6 with fixes
and tests for unbundled builds.

Bug: 80428539
Test: java_test.go
Change-Id: Ib7aaeebb93137270d7336a7293a08ed7064a3184