feat: validate package of imports (#627)

Closes partially #543

### Summary of Changes

* Show an error if a package is imported that does not exist
* Show a warning if an empty package is imported

src/language/validation/other/imports.ts

@@ -0,0 +1,30 @@
+import { ValidationAcceptor } from 'langium';
+import { SdsImport } from '../../generated/ast.js';
+import { SafeDsServices } from '../../safe-ds-module.js';
+import { isEmpty } from 'radash';
+
+export const CODE_IMPORT_MISSING_PACKAGE = 'import/missing-package';
+export const CODE_IMPORT_EMPTY_PACKAGE = 'import/empty-package';
+
+export const importPackageMustExist =
+    (services: SafeDsServices) =>
+    (node: SdsImport, accept: ValidationAcceptor): void => {
+        if (!services.workspace.PackageManager.hasPackage(node.package)) {
+            accept('error', `The package '${node.package}' does not exist.`, {
+                node,
+                property: 'package',
+            });
+        }
+    };
+
+export const importPackageShouldNotBeEmpty =
+    (services: SafeDsServices) =>
+    (node: SdsImport, accept: ValidationAcceptor): void => {
+        const declarationsInPackage = services.workspace.PackageManager.getDeclarationsInPackage(node.package);
+        if (isEmpty(declarationsInPackage)) {
+            accept('warning', `The package '${node.package}' is empty.`, {
+                node,
+                property: 'package',
+            });
+        }
+    };

src/language/validation/other/modules.ts

@@ -3,9 +3,7 @@ import { isSdsDeclaration, isSdsPipeline, isSdsSegment, SdsModule } from '../../
 import { isInPipelineFile, isInStubFile } from '../../helpers/fileExtensions.js';
 
 export const CODE_MODULE_MISSING_PACKAGE = 'module/missing-package';
-
 export const CODE_MODULE_FORBIDDEN_IN_PIPELINE_FILE = 'module/forbidden-in-pipeline-file';
-
 export const CODE_MODULE_FORBIDDEN_IN_STUB_FILE = 'module/forbidden-in-stub-file';
 
 export const moduleWithDeclarationsMustStatePackage = (node: SdsModule, accept: ValidationAcceptor): void => {

src/language/validation/safe-ds-validator.ts

@@ -72,6 +72,7 @@ import {
     lambdaParametersMustNotBeAnnotated,
 } from './other/declarations/annotationCalls.js';
 import { memberAccessMustBeNullSafeIfReceiverIsNullable } from './other/expressions/memberAccesses.js';
+import { importPackageMustExist, importPackageShouldNotBeEmpty } from './other/imports.js';
 
 /**
  * Register custom validation checks.
@@ -118,6 +119,7 @@ export const registerValidationChecks = function (services: SafeDsServices) {
         SdsEnumVariant: [enumVariantMustContainUniqueNames, enumVariantParameterListShouldNotBeEmpty],
         SdsExpressionLambda: [expressionLambdaMustContainUniqueNames],
         SdsFunction: [functionMustContainUniqueNames, functionResultListShouldNotBeEmpty],
+        SdsImport: [importPackageMustExist(services), importPackageShouldNotBeEmpty(services)],
         SdsIndexedAccess: [indexedAccessesShouldBeUsedWithCaution],
         SdsLambda: [lambdaParametersMustNotBeAnnotated, lambdaParameterMustNotHaveConstModifier],
         SdsMemberAccess: [

tests/resources/validation/other/imports/empty.sdstest

@@ -0,0 +1 @@
+package tests.other.imports.empty

tests/resources/validation/other/imports/main with issues.sdstest

@@ -0,0 +1,15 @@
+package tests.other.imports
+
+// $TEST$ error "The package 'tests.other.imports.missing' does not exist."
+from »tests.other.imports.missing« import *
+// $TEST$ error ""The package 'tests.other.imports.missing' does not exist."
+from »tests.other.imports.missing« import C
+// $TEST$ error ""The package 'tests.other.imports.missing' does not exist."
+from »tests.other.imports.missing« import C as D
+
+// $TEST$ warning "The package 'tests.other.imports.empty' is empty."
+from »tests.other.imports.empty« import *
+// $TEST$ warning "The package 'tests.other.imports.empty' is empty."
+from »tests.other.imports.empty« import C
+// $TEST$ warning "The package 'tests.other.imports.empty' is empty."
+from »tests.other.imports.empty« import C as D

tests/resources/validation/other/imports/main without issues.sdstest

@@ -0,0 +1,8 @@
+package tests.other.imports
+
+// $TEST$ no error r"The package '\.*' does not exist\."
+// $TEST$ no warning r"The package '\.*' is empty\."
+
+from tests.other.imports.nonEmpty import *
+from tests.other.imports.nonEmpty import C
+from tests.other.imports.nonEmpty import C as D

tests/resources/validation/other/imports/non empty.sdstest

@@ -0,0 +1,3 @@
+package tests.other.imports.nonEmpty
+
+class C