Support for implicit package prefixes

Now, if all Kotlin files directly under a source root have the same
package statement, it's used as an implicit package prefix for that
source root. It's honored by the package name/directory name mismatch
inspection and by new file creation actions;
support in refactorings is TODO.

 #KT-17306 Fixed

idea/tests/org/jetbrains/kotlin/idea/caches/ImplicitPackagePrefixTest.kt

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.caches
+
+import com.intellij.psi.PsiDocumentManager
+import com.intellij.testFramework.LightProjectDescriptor
+import org.jetbrains.kotlin.idea.test.KotlinLightCodeInsightFixtureTestCase
+import org.jetbrains.kotlin.idea.test.KotlinLightProjectDescriptor
+import org.jetbrains.kotlin.idea.util.application.executeWriteCommand
+import org.jetbrains.kotlin.idea.util.sourceRoots
+
+class ImplicitPackagePrefixTest : KotlinLightCodeInsightFixtureTestCase() {
+    private lateinit var cacheService: PerModulePackageCacheService
+
+    override fun getProjectDescriptor(): LightProjectDescriptor = KotlinLightProjectDescriptor.INSTANCE
+
+    override fun setUp() {
+        super.setUp()
+        cacheService = PerModulePackageCacheService.getInstance(myFixture.project)
+    }
+
+    private fun prefix(): String {
+        return cacheService.getImplicitPackagePrefix(myFixture.module.sourceRoots[0]).asString()
+    }
+
+    fun testSimple() {
+        myFixture.configureByText("foo.kt", "package com.example.foo")
+        assertEquals("com.example.foo", prefix())
+    }
+
+    fun testAmbiguous() {
+        myFixture.configureByText("foo.kt", "package com.example.foo")
+        myFixture.configureByText("bar.kt", "package com.example.bar")
+        assertEquals("", prefix())
+    }
+
+    fun testUpdateOnCreate() {
+        myFixture.configureByText("foo.kt", "package com.example.foo")
+        assertEquals("com.example.foo", prefix())
+        myFixture.configureByText("bar.kt", "package com.example.bar")
+        assertEquals("", prefix())
+    }
+
+    fun testUpdateOnDelete() {
+        myFixture.configureByText("foo.kt", "package com.example.foo")
+        val bar = myFixture.configureByText("bar.kt", "package com.example.bar")
+        assertEquals("", prefix())
+        myFixture.project.executeWriteCommand("") {
+            bar.delete()
+        }
+        assertEquals("com.example.foo", prefix())
+    }
+
+    fun testUpdateOnChangeContent() {
+        val foo = myFixture.configureByText("foo.kt", "package com.example.foo")
+        assertEquals("com.example.foo", prefix())
+        myFixture.project.executeWriteCommand("") {
+            foo.viewProvider.document!!.let {
+                it.replaceString(0, it.textLength, "package com.example.bar")
+                PsiDocumentManager.getInstance(project).commitDocument(it)
+            }
+        }
+        assertEquals("com.example.bar", prefix())
+    }
+}