[jdk15u-dev] RFR: 8280373: Update Xalan serializer / SystemIDResolver to align with JDK-8270492

[Olga Mikhaltsova]

I'd like to backport JDK-8280373 to jdk15u in order to align after JDK-8270492 the copy 
`src/java.xml/share/classes/com/sun/org/apache/xml/internal/serializer/utils/SystemIDResolver.java`
to its origin
`src/java.xml/share/classes/com/sun/org/apache/xml/internal/utils/SystemIDResolver.java`
All regular tests passed.

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Commit messages:
 - Backport c180070cb59b8e075376ae913c5db9a4ed868303

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=170&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280373
  Stats: 5 lines in 1 file changed: 2 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/170.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/170/head:pull/170

PR: https://git.openjdk.java.net/jdk15u-dev/pull/170