#66: Resolve imports relative to file's location

Part II: resolve relative import references (first part was
about type references).

Legend:

Many projects are storing proto files in a locations that are not marked as source/resource root.
As a result, imports are highlighted with error "file does not exist", and all imported types are not resolvable.

Plugin can try to resolve imports relative to source file's location using following rules:

1. If `package` is not set, try to look in the same folder where source file is.
2. If package is set, try to look from parent folder.
3. Parent folder should be computed as source file's folders, with removed corresponding parts of the package.

For example:

`/home/user/project/foo/bar/hello.proto`:

```proto
package foo.bar;
import "baz/import.proto";
```

Import should be resolved to `/home/user/project/baz/import.proto`.

Author: kshchepanovskyi

src/main/java/io/protostuff/jetbrains/plugin/reference/file/FilePathReferenceProvider.java

@@ -102,6 +102,11 @@ public PsiReference[] getReferencesByElement(@NotNull PsiElement element,
                                                  int offset,
                                                  final boolean soft,
                                                  @NotNull final Module... forModules) {
+        PsiElement tmp = element;
+        while (!(tmp instanceof ProtoPsiFileRoot || tmp == null)) {
+            tmp = tmp.getParent();
+        }
+        ProtoPsiFileRoot root = (ProtoPsiFileRoot) tmp;
         return new FileReferenceSet(text, element, offset, this, true, myEndingSlashNotAllowed) {
 
 
@@ -132,12 +137,12 @@ public Collection<PsiFileSystemItem> computeDefaultContexts() {
                 if (forModules.length > 0) {
                     Set<PsiFileSystemItem> rootsForModules = ContainerUtil.newLinkedHashSet();
                     for (Module forModule : forModules) {
-                        rootsForModules.addAll(getRoots(forModule, null));
+                        rootsForModules.addAll(getRoots(forModule, root));
                     }
                     return rootsForModules;
                 }
 
-                return getRoots(ModuleUtilCore.findModuleForPsiElement(getElement()), null);
+                return getRoots(ModuleUtilCore.findModuleForPsiElement(getElement()), root);
             }
 
             @Override

src/test/java/io/protostuff/jetbrains/plugin/reference/ReferenceTest.java

@@ -4,6 +4,7 @@
 import com.intellij.psi.PsiReference;
 import com.intellij.testFramework.fixtures.LightCodeInsightFixtureTestCase;
 import io.protostuff.jetbrains.plugin.psi.DataType;
+import io.protostuff.jetbrains.plugin.psi.ProtoPsiFileRoot;
 
 /**
  * Tests for resolving type references.
@@ -54,6 +55,18 @@ public void testImportedRelativelyTwoLevelsMessageReference() {
                 "reference/relative/import/ImportedRelativelyTestData.proto");
     }
 
+    public void testImportedRelativelyFileReference() {
+        String[] file = new String[]{"reference/relative/import/ImportedRelativelyFileReferenceTestData.proto",
+                "reference/relative/import/ImportedRelativelyTestData.proto"};
+        myFixture.configureByFiles(file);
+        PsiReference referenceAtCaretPosition = myFixture.getReferenceAtCaretPositionWithAssertion(file);
+        PsiElement target = referenceAtCaretPosition.resolve();
+        assertNotNull(target);
+        assertTrue(target instanceof ProtoPsiFileRoot);
+        ProtoPsiFileRoot protoPsiFileRoot = (ProtoPsiFileRoot) target;
+        assertEquals("ImportedRelativelyTestData.proto", protoPsiFileRoot.getName());
+    }
+
     /**
      * Check that type references work correctly inside of group block.
      *

src/test/resources/reference/relative/import/ImportedRelativelyFileReferenceTestData.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+package relative.import;
+
+import "relative/import/<caret>ImportedRelativelyTestData.proto";
+
+message TestMessage {
+    import.ImportedMessage1 message = 1;
+}