Add support for top-level Kotlin functions #847

[volivan239]

Description

With these changes, UtBot is able to generate tests for top-level Kotlin functions including extension functions. There are two main subjects that were changed:

1. In plugin, we have to look for KtFiles and their FacadeClasses among the other classes that we were already looking for. Note that as Kotlin reflection is not fully supported for such classes (see issue), some pieces of code were rewritten to use Java reflection.
2. In Kotlin renderer, we should omit calling to classes like MainKt when rendering top-level function calls. In order to support this, method ClassId.isKotlinFile was added, which checks kind in Kotlin's @Metadata annotation (see docs for more info). Also, for extension functions we should replace fictive caller with extension receiver, borrowing it from arguments (see comment to CgMethodCall.takeCallerFromArgumentsIfNeeded() for more detials).

Alongside with above changes in plugin and codegen, following side-changes were made to handle exploited problems:

1. Fixed inconsistency between KClass<*>.signature() and MemberInfo.signature()
2. Fixed name in builtin-classids timeunitClassId and durationClassId
3. KCallable<*>.declaringClazz and MemberInfo.paramNames were slightly changed to work properly when given function is an extension function

Fixes #847

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Automated Testing

Added tests to org/utbot/examples/codegen/FileWithTopLevelFunctionsTest.kt

Manual Scenario

Checked on some top-level methods (including fun main() in main class) -- generates correct tests both in Java and Kotlin

Checklist (remove irrelevant options):

• The change followed the style guidelines of the UTBot project
• Self-review of the code is passed
• The change contains enough commentaries, particularly in hard-to-understand areas
• New documentation is provided or existed one is altered
• No new warnings
• New tests have been added
• All tests pass locally with my changes

[volivan239]

Note that this doesn't resolve all problems featured by #847, as currently this doesn't work for extension functions. Needs a little bit more investigation and additional commit/PR

[volivan239]

UPD. Now extension functions should also work fine, see PR description for details