Add more diagnostic info

Resolver could be GCed if there is no any valid hard reference to it.
Some impl of PackageFragmentProvider (like DelegatingPackageFragmentProvider)
keep hard ref to resolver.
For the sake of investigation it is worth to know what kind of PFP is there.

#EA-725255

Merge-request: KT-MR-8581
Merged-by: Vladimir Dolzhenko <Vladimir.Dolzhenko@jetbrains.com>

core/descriptors/src/org/jetbrains/kotlin/descriptors/impl/ModuleDescriptorImpl.kt

@@ -155,8 +155,12 @@ class ModuleDescriptorImpl @JvmOverloads constructor(
     override fun <T> getCapability(capability: ModuleCapability<T>) = capabilities[capability] as? T
 
     override fun toString(): String {
-        val toString = super.toString()
-        return if (isValid) toString else "$toString !isValid"
+        return buildString {
+            append(super.toString())
+            if (!isValid) append(" !isValid")
+            append(" packageFragmentProvider: ")
+            append(packageFragmentProviderForModuleContent?.javaClass?.simpleName)
+        }
     }
 }