be-py: put a dashed library's own directory on PYTHONPATH

[notactuallytreyanastasio]

temper run -b py on a library named hello-world that only prints
fails:

PYTHONPATH=.../temper.out/py/hello_world:.../temper.out/py/temper-core:.../temper.out/py/deps
ModuleNotFoundError: No module named 'hello_world'

The package is at temper.out/py/hello-world/hello_world/, so the
directory that has to be on the path is hello-world. runPyBestEffort
built the library's path from its Python module name, hello_world,
which names no directory. The dependency paths next to it use the
Temper name, and they cover every library in
transitiveDependencies, so the run worked whenever the build had
recorded a dependency from the library on another one: an Int
addition, which uses temper-core's int_add, was enough. A library
that only calls console.log records none, and a name with no dash
hid the mismatch because the two names are the same.

The library's own path now uses the Temper name too. Its directory is
then also the one the dependency loop skips, so it is listed once.

RunTest.runPyBackendDashedName runs a hello-world library that
prints one line. Before this change it fails with the
ModuleNotFoundError above; after, it prints I ran.

Fixes #609.

Reproduction: 56-py-run-hyphenated-library/ in temper-issue-repros.

🤖 Generated with Claude Code

[mikesamuel]

fyi, we use the header here for the name if it's not specified, so

# Hello World

would be an equivalent config.

[mikesamuel]

Could this be a when (request) instead of using as? so that we explicitly cover the different request cases. It's ok if we're using null for some cases, just it's clearer if we're deriving this that we've covered our bases.