Use decoded names when checking if a classfile is reachable from client code

Operators are encoded into `$<plain-name>` during compilation to please the
JVM. Hence, the former implementation of `PackageInfo.isAccessible` was broken
as it relied on the encoded name of a class, instead of using the decoded one.
This commit fixes the issue.

Fixes #46

review by @gkossakowski, @retronym