RFR: JDK-8208608: Update --module-source-path to allow explicit source paths for specific modules

[Jonathan Gibbons]

Please review a moderately simple update to the --module-source-path option
supported directly by javac and thus indirectly by javadoc.

The change is to allow a new form, similar to that used for the 
--patch-module
option, which allows an explicit source path to be provided for any 
specific module,
without having to use the existing pattern-based syntax.

The CSR is in review.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8208608
CSR: https://bugs.openjdk.java.net/browse/JDK-8208609
Webrev: http://cr.openjdk.java.net/~jjg/8208608/webrev.00/