Double // in file paths

Problem/Motivation

This is a follow up to #2925899: MigrateUpgradeImportBatch does not use source_private_file_path & source_base_path correctly, making it impossible to have public & private files in separate locations.
benjifisher reported

When I run file migrations and there are missing files, the message often lists a path including //, and I think one of those characters comes from this line. Do we need to add a trailing / here?

$configuration['source']['constants']['source_base_path'] = rtrim($base_path, '/') . '/';

Proposed resolution

TBD

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet