Absolutized paths involving the scala package.

Confusing, now-it-happens now-it-doesn't mysteries lurk in the darkness. When
scala packages are declared like this:

  package scala.collection.mutable

Then paths relative to scala can easily be broken via the unlucky presence of
an empty (or nonempty) directory. Example:
```
  // a.scala
 package scala.foo
 class Bar { new util.Random }

 % scalac ./a.scala
 % mkdir util
 % scalac ./a.scala
 ./a.scala:4: error: type Random is not a member of package util
   new util.Random
            ^
 one error found
```
There are two ways to play defense against this:

- don't use relative paths; okay sometimes, less so others
- don't "opt out" of the scala package

This commit mostly pursues the latter, with occasional doses of the former.

I created a scratch directory containing these empty directories:

  actors annotation ant api asm beans cmd collection compat
 concurrent control convert docutil dtd duration event factory
 forkjoin generic hashing immutable impl include internal io
 logging macros man1 matching math meta model mutable nsc parallel
 parsing partest persistent process pull ref reflect reify remote
 runtime scalap scheduler script swing sys text threadpool tools
 transform unchecked util xml

I stopped when I could compile the main src directories even with all those
empties on my classpath.