Tighten up sys.path, and use absolute imports everywhere.

For convenience, we had added the folder that dotest.py was in
to sys.path, so that we could easily write things like
`import lldbutil` from anywhere and any test.  This introduces
a subtle problem when using Python's package system, because when
unittest2 imports a particular test suite, the test suite is detached
from the package.  Thus, writing "import lldbutil" from dotest imports
it as part of the package, and writing the same line from a test
does a fresh import since the importing module was not part of
the same package.

The real way to fix this is to use absolute imports everywhere.  Instead
of writing "import lldbutil", we need to write "import
lldbsuite.test.util".  This patch fixes up that and all other similar
cases, and additionally removes the script directory from sys.path
to ensure that this can't happen again.

llvm-svn: 251886