Better diagnostics for conflicting overloads.

Skip declarations without sources in reporting, not when determining redeclaration groups:
this allows emitting informative diagnostics for incremental compilation.
Provide containing declaration with "kind", e.g., "package '<root>'", "class A", and so on.

jps-plugin/testData/incremental/classHierarchyAffected/secondaryConstructorAdded/build.log

@@ -5,7 +5,7 @@ Compiling files:
 src/A.kt
 End of files
 COMPILATION FAILED
-'public constructor A(x: kotlin.String)' conflicts with another declaration: public constructor A(x: kotlin.String)
+'public constructor A(x: kotlin.String)' conflicts with another declaration in package '<root>'
 
 
 Cleaning output files:

jps-plugin/testData/incremental/pureKotlin/funRedeclaration/build.log

@@ -6,4 +6,4 @@ Compiling files:
 src/fun2.kt
 End of files
 COMPILATION FAILED
-'public fun function(): kotlin.Unit' conflicts with another declaration: public fun function(): kotlin.Unit
+'public fun function(): kotlin.Unit' conflicts with another declaration in package 'test'

jps-plugin/testData/incremental/pureKotlin/funVsConstructorOverloadConflict/build.log

@@ -6,4 +6,4 @@ Compiling files:
 src/fun2.kt
 End of files
 COMPILATION FAILED
-'public constructor function()' conflicts with another declaration: public constructor function()
+'public constructor function()' conflicts with another declaration in package 'test'