Review and refactor Path::strip_prefix usages

[messense]

It's getting quite messy, it needs both input to be relative/absolute to work correctly, but we do not have that guarantee at the moment. Things can easily go wrong, for example #1198.

Current state as of 5f2c219

rg strip_prefix
src/compile.rs
508:                        if py_init == sym_name.strip_prefix('_').unwrap_or(&sym_name) {
516:                            if py_init == sym_name.strip_prefix('_').unwrap_or(sym_name) {

src/python_interpreter/mod.rs
545:                                python.strip_prefix("python").unwrap_or(python)

src/module_writer.rs
662:                .strip_prefix(python_module.parent().unwrap())?;
732:            .strip_prefix(python_module.parent().unwrap())?;
842:        let relative = absolute.strip_prefix(python_module.as_ref().parent().unwrap())?;
939:                    let relative = file.path().strip_prefix(data.parent().unwrap())?;

src/auditwheel/platform_tag.rs
106:                if let Some(value) = value.strip_prefix("musllinux_") {
118:                    let value = value.strip_prefix("manylinux_").unwrap_or(&value);

src/source_distribution.rs
284:                    .strip_prefix(&pyproject_dir)
326:            .strip_prefix(pyproject_dir)?
345:        let relative_manifest_path = abs_manifest_path.strip_prefix(pyproject_dir)?;
485:            cargo_lock_path.strip_prefix(project_root)?
487:            cargo_lock_path.strip_prefix(&abs_manifest_dir)?
515:            let target = root_dir.join(source.strip_prefix(&pyproject_dir)?);
545:                let target = root_dir.join(&source.strip_prefix(pyproject_dir)?);

src/build_options.rs
996:        let (python_impl, python_ver) = if let Some(ver) = python.strip_prefix("pypy") {
997:            (InterpreterKind::PyPy, ver.strip_prefix('-').unwrap_or(ver))
998:        } else if let Some(ver) = python.strip_prefix("python") {
1001:                ver.strip_prefix('-').unwrap_or(ver),

Most of them are fine, just need to check source_distribution.rs and module_writer.rs.