Support for adding license to files whose name has suffix ".p4"

[jafingerhut]

Files whose names have the suffix '.p4" are typically written in the P4 programming language: https://p4.org

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[jafingerhut]

@smolkaj @jonathan-dilorenzo These people working at Google work on projects that use the P4 programming language.

[flwyd]

Thanks for contributing to addlicense! Please sync your pull request with master; I've changed filetypes to one-per-line to make future merge conflicts less likely. Please also add a test case in the testdata/initial and testdata/expected directories.

[jafingerhut]

The error messages I see in the log for the failing "linter / lint" CI test appear to have little to do with my 1-line change, which could not have possibly by itself introduced so many linter errors.

[flwyd]

Please add test cases with a before and after file in the testdata directory, and add a case to TestLicenseHeader in main_test.go.

I've fixed the linter errors; another sync with the master branch might be needed to pick up the new workflow definition.