RFR: JDK-8280745: Allow SuppressWarnings to be used in package declarations

[Joe Darcy]

The SuppressWarnings annotation type is declared have multiple targets, including modules; however, "package" is left off of its target list. As package-info file are another kind of declaration where a compiler could give warnings, allowing SuppressWarnings in that case is reasonable. Wanting SuppressWarnings in package-info file came up while working on doclint warnings (JDK-8280534). 

Please also review the companion CSR:
https://bugs.openjdk.java.net/browse/JDK-8280745

While the SuppressWarnings annotation does have a JLS section (9.6.4.5. @SuppressWarnings), the section doesn't mention the target list therefore doesn't appear to need to be updated.

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Commit messages:
 - JDK-8280745: Allow SuppressWarnings to be used in package declarations

Changes: https://git.openjdk.java.net/jdk/pull/7239/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7239&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280745
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7239.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7239/head:pull/7239

PR: https://git.openjdk.java.net/jdk/pull/7239