Generate error for classes without package.

The implementation of zinc rebase/split/merge requires that all scala
source files have declared packages. In the current code, when a user
builds a scala source file without a package declaration, pants crashes
and dumps its stack with a "No such file or directory" error.

This change adds code to detect when a compilation produced a class
file without a package, and generates a comprehensible error message.

Auditors: benjy
Remove debugging print.
Remove temp analysis files.

Pants was leaving temporary analysis files, which was clogging up the
tmp directories on dev machines.

Auditors: benjy
Correct fix for undeleted analysis files.

In scala compilation, pants generated temporary files during
the process of merging, splitting, and rebasing analysis files.
These files weren't being cleaned up correctly.

This change fixes the two reasons for the lingering files:
(1) Zinc generates two kinds of analysis files: java serialization files,
  and textual relations files. In some operations, the serialization files
  were being cleaned up, but the textual relations files.
(2) In some cases, the automatic temp file cleanup used for
  removing the analysis files when they are no longer needed
  was disabled.

In manually observed test runs, no analysis files were left
after pants completed.

Auditors: benjy
Cc: tdyas
Merge branch 'zinc_tmp_cleanup'