Improve error reporting for compiling against package not visible due to modules

Executive summary: Lying error messages are the worst
time-wasters, especially if they are lying about complex,
scaled-up configuration information.

When working in modular code, if I attempt to import a package
which is not visible to my package, sometimes the error message
just bluntly states that the package does not exist, even if the
package *does* exist but cannot be accessed due to details of
the module system.

This has wasted hours of my time today, because I was treating
the "package does not exist" statement as correct, and therefore
knocked my head against a wall trying to find the configuration
error which was causing my package to "not exist". (It was made
more difficult of course cause the configuration was set up by an
IDE, under masses of properties, config. dialogs, ant files, class
paths, etc., etc. Much of my time was cutting down to a minimal
reproducible case.) Eventually I found that, yes, my package is
correctly made available (on javac -classpath, as it happens, but
it might also have been somewhere else more "modern"). So
the error message was a lie, and due to a very simple root cause:
The module I was working in does not read the (unnamed) module
containing the package I was importing–which "does not exist"
according to the lying error message–but which really exists and
is not readable. Due to a simple programmer error.

This is either an incomplete fix of JDK-8169197, or a closely
related failure mode. Let's get these messages right, please.

See below for an example.

--------
        jar -tvf lib/testng.jar org/testng/annotations/Test.class
  1535 Tue Sep 26 08:53:24 PDT 2017 org/testng/annotations/Test.class
--------
        javac -d out --patch-module java.base=src -cp lib/testng.jar src/test0.java
+ javac -d out --patch-module java.base=src -cp lib/testng.jar src/test0.java
src/test0.java:5: error: package java.sql is not visible
import java.sql.*;
           ^
  (package java.sql is declared in module java.sql, but module java.base does not read it)
src/test0.java:9: error: package org.testng.annotations does not exist
import org.testng.annotations.*;
^
2 errors
--------
        cat src/test0.java
// javac -d out --patch-module java.base=src -cp lib/testng.jar src/test0.java

package java.lang.invoke;

import java.sql.*;
//error: package java.sql is not visible
// (package java.sql is declared in module java.sql, but module java.base does not read it)

import org.testng.annotations.*;
//error: package org.testng.annotations does not exist

// BUG: THE MESSAGE SHOULD SAY SOMETHING LIKE THIS:
//error: package org.testng.annotations is not visible
// (package org.testng.annotations is declared in the unnamed module, but java.base does not read it)

class test0 {
  // @Test public void test() {...}
}
--------