External plugin dependency files present in plugin sandbox

[baron1405]

External plugin dependencies that are not packaged as JAR files, can result in files from that dependency leaking into the plugin sandbox. To reproduce:

1. Create a plugin
2. Add an external dependency on CheckStyle-IDEA 5.1.4 (or whatever the current version)
3. Build the plugin
4. Notice that various files from the CheckStyle-IDEA plugin are present in the build/idea-sandbox/plugins/<plugin name>/lib directory

From debugging the code, it appears that the external dependency classes and META-INF directories are reported by Gradle with a trailing /. whereas the plugin dependencies directories in librariesToIgnore do not have the trailing slash. So the exclusion fails and the artifacts are left in the plugin bundle.