RFR: 8284942: Proxy building can just iterate superinterfaces once

[liach]

Currently, in ProxyBuilder::mapToModule and ProxyBuilder::defineProxyClass, the interfaces are iterated twice. The two passes can be merged into one, yielding the whole proxy definition context (module, package, whether there's package-private interface) when determining the module.

Split from #8278. Helpful for moving proxies to hidden classes, but is a good cleanup on its own.

-------------

Commit messages:
 - 8284942: Proxy building can just iterate superinterfaces once

Changes: https://git.openjdk.java.net/jdk/pull/8281/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8281&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284942
  Stats: 64 lines in 1 file changed: 16 ins; 25 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8281.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8281/head:pull/8281

PR: https://git.openjdk.java.net/jdk/pull/8281