pre-upload: fix chdir/output finishing when running hooks

The current _run_project_hooks helper will call os.chdir but then
return early before restoring its state to the previous location.
To fix this, rip the core of _run_project_hooks into a new helper
called _run_project_hooks_in_cwd, and then wrap that call in a try
block so we can put the chdir call into the finally section.

We have a similar problem with the output helper where we don't
guarantee calling output.finish() before returning.  To solve that,
we have to rework the Output API slightly by delaying the setup of
the num_hooks method to the point where we can calculate it.

Bug: 124462370
Test: `repo upload` still works and summarizes across multiple repos
Change-Id: I18780bcfe91db1cd276d7bdfdfd62badfc83a0d7