SI-7335 Mandate that parents of Predef must be defined in Predef.scala

This avoids thorny issues of cyclic references and/or missing symbol errors
due to the way Predef is completed earlier than user classes, and due to the
addition of `import Predef._` to LowPriorityImplicits.scala. More details in
Lukas's comments in [2].

How did we bootstrap originally? Manually ordering source files,
either in the Ant Build [1], or later in Global [2]. But neither mechanism
remains in place (due, I suppose, to the difficulty in defending them
with a test case.) The reordering in Global was removed in [3].

After the change, we can compile the standard library without the
results of a prior compilation run on the classpath.

    rm -rf target/locker/library/classes/scala && export LIB=build/pack/lib; java -Xmx1G -cp $LIB/scala-library.jar:$LIB/scala-reflect.jar:$LIB/scala-compiler.jar:$LIB/forkjoin.jar scala.tools.nsc.Main @args.txt
    src/library/scala/SerialVersionUID.scala:15: warning: Implementation restriction: subclassing Classfile does not
    make your annotation visible at runtime.  If that is what
    you want, you must write the annotation class in Java.
    class SerialVersionUID(value: Long) extends scala.annotation.ClassfileAnnotation
          ^
    warning: there were 75 deprecation warning(s); re-run with -deprecation for details
    warning: there were 1 unchecked warning(s); re-run with -unchecked for details
    warning: there were 5 feature warning(s); re-run with -feature for details
    four warnings found

 Where @args.txt contains:

     -classpath
     lib/forkjoin.jar:target/locker/library/classes
     -sourcepath
     src/library
     -d
     target/locker/library/classes
     src/library/scala/Function3.scala
     src/library/scala/Console.scala
     <more of src/library/**, unsorted>

This change will break scala-virtualized, which will need to move
the contents of StandardEmbeddings.scala into Predef.scala.

[1] https://github.com/scala/scala/commit/c5441dc
[2] https://github.com/scala/scala/commit/e72f0c7
[3] https://github.com/scala/scala/pull/968
SI-7335 Don't import Predef._ in Predef.scala

The check for this was foiled by the introduction of
DeprecatedPredef in c26a8db0. This didn't cause any
harm, but in any case this commit restores the intended
state of affairs.

The change is visible only through logging; no test case
is included.

    qbin/scalac -Ydebug -Ylog:all src/library/scala/Predef.scala -Ystop-after:typer 2>&1 | grep -i "Omitted import of Predef"
    [log namer] Omitted import of Predef._ for Predef.scala
    [log typer] Omitted import of Predef._ for Predef.scala

A redundant override of `firstDefinesClassOrObject` is also
removed.

Maybe we can remove the special case altogether?
SI-7335 Add logging for a now-impossible* case in Symbol#exists.

* Impossible (to the best of my knowledge) because LowPriorityImplicits
is now defined in Predef.scala. Were I more sure, we could trip an
assertion here, rather than the devWarning.
SI-7335 Sharpen up comment about implicit prioritization.