RFR: 8300204: Sealed-class hierarchy graph missing nodes

[Chen Liang]

Please review this simple patch that fixes the package determination for nested classes in the `@sealedGraph` taglet.

Current JDK 21:
https://download.java.net/java/early_access/jdk21/docs/api/java.base/java/lang/foreign/MemoryLayout.html
https://download.java.net/java/early_access/jdk21/docs/api/java.base/java/lang/foreign/ValueLayout.html
![image](https://user-images.githubusercontent.com/7806504/236984282-f4361510-6208-4469-92d6-b9ce143204c3.png)

This patch:
https://cr.openjdk.org/~liach/8300204/java.base/java/lang/foreign/MemoryLayout.html
https://cr.openjdk.org/~liach/8300204/java.base/java/lang/foreign/ValueLayout.html
![image](https://user-images.githubusercontent.com/7806504/236984331-a877a493-487a-444a-984c-9175393cbb92.png)

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Commit messages:
 - 8300204: Sealed-class hierarchy graph missing nodes

Changes: https://git.openjdk.org/jdk/pull/13875/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13875&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300204
  Stats: 9 lines in 1 file changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13875.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13875/head:pull/13875

PR: https://git.openjdk.org/jdk/pull/13875