RFR: JDK-8246705: javadoc gives "misleading" and incomplete warning message.

[Jonathan Gibbons]

Please review a small change to a poor warning message that is generated 
in unusual circumstances. The circumstances are caused by using 
--expand-requires and not having the source available for all the modules.

  * The message incorrectly uses the simple name of a module instead of
    the fully qualified name.
    The fix is trivial: use the fully qualified name

  * The message is too broad and says "module not found" when it really
    means "source for module not found"
    The message is improved

  * The message may be unnecessarily repeated.
    To minimize risk of a complicated solution, a simple cache is used
    to track instances of the warnings to avoid duplicates.

In addition, a couple of inappropriate trailing periods are removed from 
a couple of related messages.

Two tests are updated; one test is marked for this bug.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8246705
Webrev: http://cr.openjdk.java.net/~jjg/8246705/webrev.00/

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