[lldb] Unconditionally strip the `.py(c)` extension when loading a module

Currently we only strip the Python extension when the file exists on
disk because we assumed that if it didn't exist it was a module.
However, with the change from D89334 this is no longer the case as we
want to be able to import a relative path to a .py as a module. Since we
always import a scripting module as a "python module" we should always
strip the extension if present.

Differential revision: https://reviews.llvm.org/D89352