REVIEW-2549: renamed BuildableModelElement.dependsOn() -> builtBy()
Split C and Assembler support out of CppPlugin

- [C/Cpp/Assembler]LangPlugin no longer add compile/assemble tasks for native binaries
- Added CPlugin and AssemblerPlugin for adding full language + native support
- Added string ids for more language plugins
DSL docs for language source sets
Fixed sample for api change
Fix test on UnknownOS