GitLab

* We've returned to the old package layout, so fix those paths.
* Be more specific about which sources/* directories contain modules
  for `import-module`.
* Include hint about `--system windows64`.
* Reword the `--package` vs `--no-package` section to encourage using
  the default behavior.
* Fix the bit about `--toolchain clang` for `run-all.py`. Since Clang
  is now the default, we need to advise about using GCC.
* Remove the bit about libc++ ARM32 unwinding being broken, since
  I've fixed that now.

Change-Id: I61b8ca9458c1884a38248c8c04dc6bc529781570