Fix import on directory

Earler https://rbcommons.com/s/twitter/r/4386 removed the usage of `com.twitter.intellij.pants.util.PantsUtil#convertToTargetSpecs` which implicitly convert an empty target spec (in the case where user chooses a directory instead of a specifc target) to `<selected folder>::`. This RB makes the directory case explicit as one of the GUI options.

Other fix:
* Target in GUI will show up correctly as `a/b/c::` instead of `Optional[a/b/c::]`

Testing Done:
https://travis-ci.org/pantsbuild/intellij-pants-plugin/builds/176898711

Reviewed at https://rbcommons.com/s/twitter/r/4389/

common/com/twitter/intellij/pants/util/PantsUtil.java

@@ -509,8 +509,8 @@ public static <K, V> Map<K, V> filterByValue(Map<K, V> map, Condition<V> conditi
     return result;
   }
 
-  public static Optional<String> getRelativeProjectPath(@NotNull File projectFile) {
-    final Optional<File> buildRoot = findBuildRoot(projectFile);
+  public static Optional<String> getRelativeProjectPath(@NotNull String projectFile) {
+    final Optional<File> buildRoot = findBuildRoot(new File(projectFile));
     return buildRoot.flatMap(file -> getRelativeProjectPath(file, projectFile));
   }
 
@@ -861,7 +861,7 @@ public static List<String> convertToTargetSpecs(String importPath, List<String>
     File importPathFile = new File(importPath);
     final String projectDir =
       isBUILDFileName(importPathFile.getName()) ? importPathFile.getParent() : importPathFile.getPath();
-    final Optional<String> relativeProjectDir = getRelativeProjectPath(new File(projectDir));
+    final Optional<String> relativeProjectDir = getRelativeProjectPath(projectDir);
     // If relativeProjectDir is null, that means the projectDir is already relative.
     String relativePath = relativeProjectDir.orElse(projectDir);
     if (targetNames.isEmpty()) {

src/com/twitter/intellij/pants/settings/PantsProjectSettingsControl.java

@@ -23,6 +23,7 @@
 import com.intellij.util.ui.StatusText;
 import com.intellij.util.ui.UIUtil;
 import com.twitter.intellij.pants.PantsBundle;
+import com.twitter.intellij.pants.PantsException;
 import com.twitter.intellij.pants.util.PantsUtil;
 import org.jetbrains.annotations.NotNull;
 
@@ -97,7 +98,12 @@ public void onProjectPathChanged(@NotNull final String projectPath) {
 
     if (file.isDirectory()) {
       myTargetSpecs.setEnabled(false);
-      myTargetSpecs.setEmptyText(PantsUtil.getRelativeProjectPath(new File(file.getPath())) + "/::");
+      String text = PantsUtil.getRelativeProjectPath(file.getPath())
+                      .orElseThrow(() -> new PantsException(String.format(
+                        "Fail to find relative path from %s to build root.", file.getPath()
+                      ))) + "/::";
+      myTargetSpecs.setEmptyText(text);
+      myTargetSpecs.addItem(text, text, true);
 
       myWithDependeesCheckBox.setSelected(false);
       myWithDependeesCheckBox.setEnabled(true);