Added `@Restricted(NoExternalUse.class)` to prevent access from other packages (especially plugins).

* `Functions#printLogRecordHtml` and `Run#previousBuild` are also annotated with `@Restricted(NoExternalUse.class)` and accessed from test package.
* test package does not fail even access-modifier-checker is added to pom.xml. It seems that access-modifier-checker does not check accesses from src/test directories.