Create rule S6627: Users should not use internal APIs (#1869)

rules/S6627/kotlin/metadata.json

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+{
+  "title": "Users should not use internal APIs",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "Gradle"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6627",
+  "sqKey": "S6627",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown"
+}

rules/S6627/kotlin/rule.adoc

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+== Why is this an issue?
+
+The public API of a framework, plugin, or library is the way its provider intended it to be used.
+API stability and compatibility (within the same major version number) of a library are guaranteed only for its public API.
+
+Internal APIs are mere implementation details and are prone to breaking changes as the implementation of the library changes.
+No guarantees are being made about them. Therefore, users should not use internal APIs, even when visible.
+
+=== What is the potential impact?
+
+==== Code Stability
+
+If not fixed, your code might break when the library is upgraded to a new version, even if only the minor version number or the patch number changes.
+
+== How to fix it
+
+Replace internal API usage with the public API designed for your use case.
+This may imply a refactoring of the affected code if no one-to-one replacement is available in the public API.
+If a specific functionality is required, copying the required parts of the implementation into your code may even be better than using the internal API.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,kotlin,diff-id=1,diff-type=noncompliant]
+----
+if (org.gradle.internal.os.OperatingSystem.current().isWindows) { // Noncompliant, OperatingSystem is part of Gradle internal API
+    // ...
+}
+----
+
+==== Compliant solution
+
+[source,kotlin,diff-id=1,diff-type=compliant]
+----
+if (System.getProperty("os.name").toLowerCase().contains("windows")) { // Compliant
+    // ...
+}
+----
+
+== Resources
+
+=== Documentation
+
+* https://docs.gradle.org/current/userguide/authoring_maintainable_build_scripts.html#sec:avoiding_gradle_internal_apis[Gradle Documentation - Avoid using internal Gradle APIs]
+* https://github.com/liutikas/gradle-best-practices#dont-use-internal-apis[Best Practices when using Gradle - Don't use internal APIs (Liutikas)]

rules/S6627/metadata.json

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+{
+}