Do not suggest import keyword before package declaration

eg. `import` should not be suggested as a completion at the `|`:

```java
imp|

package dev.datho7561.utils;

public class HelloWorld {

}
```

Fixes eclipse-jdt#3799

Signed-off-by: David Thompson <davthomp@redhat.com>

Author: datho7561

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/model/CompletionTests.java

@@ -9177,7 +9177,7 @@ public void testCompletionKeywordImport7() throws JavaModelException {
 		cu.codeComplete(cursorLocation, requestor);
 
 		assertEquals(
-			"element:import    completion:import    relevance:"+(R_DEFAULT + R_RESOLVED + R_INTERESTING + R_CASE+ R_NON_RESTRICTED),
+			"",
 			requestor.getResults());
 }
 public void testCompletionKeywordImport8() throws JavaModelException {

org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/CompletionEngine.java

@@ -2164,9 +2164,12 @@ public void complete(ICompilationUnit sourceUnit, int completionPosition, int ad
 							contextAccepted = true;
 							buildContext(importReference, null, parsedUnit, null, null);
 							if(!this.requestor.isIgnored(CompletionProposal.KEYWORD)) {
-								setSourceAndTokenRange(importReference.sourceStart, importReference.sourceEnd);
-								CompletionOnKeyword keyword = (CompletionOnKeyword)importReference;
-								findKeywords(keyword.getToken(), keyword.getPossibleKeywords(), false, parsedUnit.currentPackage != null);
+								// do not suggest `import` or `package` keywords if the cursor is before an existing package declaration
+								if (parsedUnit.currentPackage == null || importReference.sourceStart >= parsedUnit.currentPackage.sourceStart) {
+									setSourceAndTokenRange(importReference.sourceStart, importReference.sourceEnd);
+									CompletionOnKeyword keyword = (CompletionOnKeyword)importReference;
+									findKeywords(keyword.getToken(), keyword.getPossibleKeywords(), false, parsedUnit.currentPackage != null);
+								}
 							}
 							debugPrintf();
 							return;