Rename `greta_python_deps()` to `greta_dep_spec()`?

[njtierney]

Not sure if one is more evocative/easier

[njtierney]

Thinking for the future, I think we could probably avoid python in the names, as we might be adding other dependencies in the future, e.g., using torch instead of tensorflow.

So python_deps arg could be deps

And the greta_python_deps() spec producer could be called greta_deps_spec()

So instead of:

install_greta_deps(python_deps = greta_python_deps())

You get

install_greta_deps(deps = greta_deps_spec())