[python/pyre.vim] Fix pyre persistent behavior (#3895)

It's necessary to provide a `-l` option to pyre with the closest parent
directory containing a `.pyre_configuration.local` file, or simply
change directory (cwd) to the root of the pyre project. Thanks to Ken
Verbosky for the code that fixes this.

Error seen when not using such a solution:

```
1031.473923 on 6: Dropping message 'ƛ Background task unexpectedly quited: Invalid configuration: Cannot find any source files to analyze. Either `source_directories` or `targets` must be specified.
```

Issue with this approach is that if you are editing files under
different projects, the `pyre persistent` process is not re-created for
each file. We have to do `:ALEStopAlllsps` in order for the process to
start with the new working directory.

Co-authored-by: Oliver Ruben Albertini <ora@fb.com>

0 files changed, 0 insertions, 0 deletions