FIR entrypoint: deduplicate some code and remove JVM dependency

Deduplicate Fir2IrExtensions creation and move the
convertToIrAndActualizeForJvm function to 'cli', which allows to remove
the dependency of 'fir.entrypoint' on 'backend.jvm'.

Note that behavior slightly changes in IncrementalFirJvmCompilerRunner:
previously the value of linkViaSignatures was always false, now it is
taken from the compiler configuration, which seems more correct.

compiler/fir/entrypoint/build.gradle.kts

@@ -21,7 +21,7 @@ dependencies {
     implementation(project(":compiler:fir:resolve"))
     implementation(project(":compiler:fir:fir-serialization"))
     implementation(project(":compiler:fir:fir2ir:jvm-backend"))
-    implementation(project(":compiler:backend.jvm"))
+    implementation(project(":compiler:ir.backend.common"))
     implementation(project(":compiler:ir.serialization.common"))
     implementation(project(":compiler:ir.serialization.jvm"))
     implementation(project(":compiler:ir.serialization.js"))