EnforceBytecodeVersion: check if the package name is valid

[aalmiray]

Apply a regex such as "^[a-z]+(.[a-z][a-z0-9])$" to check if a given classfile entry has a valid package name. Actual regex may account for uppercase characters and _ which is also valid.

The gradle-api-* JARs do not follow the MR-JAR constraints and contain entries such as org/gradle/internal/impldep/META-INF/versions/9/org/junit/platform/commons/util/ModuleUtils.class which cannot be loaded as their package name is invalid. Entries like this should not be scanned.

This particular JAR has issues (see gradle/gradle#23002) yet the fact remains that other faulty JARs may exist out there with entries that could never be loaded (unless a weird force is attempted) and thus should not cause the rul to trigger.

Related to kordamp/enforcer-gradle-plugin#27