RFR: JDK-8187974: Merge Java Mission Control 6.1 3rdpartyreame with the JDK 18.3 3rdpartyreadme

[Erik Joelsson]

When working on a closed feature, I needed access to the list of source 
files from a SetupCopyFiles call. This patch adds a specific variable in 
which this information is returned.

Webrev: http://cr.openjdk.java.net/~erikj/8187974/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8187974

/Erik