buildozer: Add parameter to substitute commands to exclude certain expressions

[Yannic]

A common use-case for buildozer 'substitute ...' is to move everything in a package to a different package (e.g. mapping packages from //... to //third_party/project/... during Copybara import). However, some packages have special meaning for Bazel and should (usually) not be renamed.

This change adds an optional last parameter to substitute and substitute_load that is used to exclude expressions if they match the passed regexp.