Improve dependency scope validation error messages for import scope

[gnodet]

Summary

This PR improves Maven's dependency scope validation error messages to provide clearer guidance when the import scope is used incorrectly.

Problem

Currently, when a user incorrectly uses import scope in a regular <dependencies> section, Maven shows a confusing error message:

'dependencies.dependency.scope' must be one of [provided, compile, runtime, test, system] but is 'import'.

This message is misleading because it suggests that import scope is never valid, when in fact it is valid in <dependencyManagement> sections with <type>pom</type>.

Solution

The PR enhances the error message to provide context-aware guidance:

'dependencies.dependency.scope' has scope 'import'. The 'import' scope is only valid in <dependencyManagement> sections.

Key Improvements:

1. Context-aware validation: Different error messages for regular dependencies vs. dependency management
2. Clear guidance: Explicitly tells users where import scope is valid
3. Focused messaging: Removes confusing list of valid scopes and focuses on the specific problem
4. Consistent implementation: Applied to both Maven 3 (compat) and Maven 4 (impl) for consistency

Changes Made

Core Changes

• Added validateDependencyScope() method in both implementations that provides context-aware error messages
• Enhanced error messages with specific guidance for import scope misuse
• Maintained backward compatibility - only error messages are improved, no functional changes

Files Modified

• compat/maven-model-builder/src/main/java/org/apache/maven/model/validation/DefaultModelValidator.java
• compat/maven-model-builder/src/test/java/org/apache/maven/model/validation/DefaultModelValidatorTest.java
• impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelValidator.java
• impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultModelValidatorTest.java

Additional Improvements

• Fixed grammar in comments (don't → not)
• Updated tests to verify the improved error messages
• Applied spotless formatting for code consistency

Example Impact

This change helps users quickly understand how to fix their Maven configuration:

❌ Incorrect usage (triggers new helpful error):

<dependencies>
  <dependency>
    <groupId>org.example</groupId>
    <artifactId>bom</artifactId>
    <version>1.0</version>
    <type>pom</type>
    <scope>import</scope>  <!-- ❌ Invalid here -->
  </dependency>
</dependencies>

✅ Correct usage:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.example</groupId>
      <artifactId>bom</artifactId>
      <version>1.0</version>
      <type>pom</type>
      <scope>import</scope>  <!-- ✅ Valid here -->
    </dependency>
  </dependencies>
</dependencyManagement>

Related Issues

Addresses the confusion reported in faktorips/faktorips.base#70

Testing

• ✅ All existing tests pass (161 tests, 0 failures)
• ✅ Updated tests verify the improved error messages
• ✅ Code compiles successfully with JDK 21
• ✅ Spotless formatting applied
• ✅ No checkstyle violations

The validation logic remains unchanged - only the error messages are enhanced to be more helpful and educational.

[cstamas]

The deprecated code and impl code is diverging (like new maven4 scopes cannot be validated with deprecated code). If this is not a concern, am ok.