Add option to export resolved mapping to a mapping file

[mknorps]

Is your feature request related to a problem? Please describe. Add an option to export mapping to a file which may later be used by user-defined mapping option developed in #260 .

This feature allows a workflow, where a user gathers mapping files from some of the existing projects to use it on another project. It could also work as a simple cache.

Describe the solution you'd like CLI option --export-mappings FILE to export the mappings computed in addition to regular --check* options.

Additional context Without this option, a user must manually edit mappings file.

A thing to consider is that user-defined mapping is always at the top of the resolvers stack. That means that if a user has a big mapping file with obsolete data, it may cause inaccuracies in matching imports and dependencies.