Separate this repo into two: a go module and the oasdiff executable

[reuvenharrison]

Oasdiff is currently a mono-repo containing a go module and the main oasdiff executable.

This structure implies two conflicting requirements:

1. General semver best-practices for the oasdiff executable
2. Golang's module versioning guidelines
   These requirements are in conflict because a breaking change to the oasdiff executable doesn't necessarily imply a breaking change to the module and vice versa.

A possible solution would be to separate this repo into two:

1. The oasdiff executable which would be practically equivalent to the current internal package.
2. A go module containing the packages like diff, flatten, checker etc.

Pros:

• Adherence to golang's module versioning guidelines and semver best-practices for the oasdiff executable
• Users of the oasdiff tool can see accurate and relevant release notes while ignoring internal implementation details of the module
• Module users can see accurate and relevant release notes excluding irrelevant changes to the oasdiff executable