lint/revive: add package doc comments

This silences all of the "should have a package comment" lint warnings
from golangci-lint.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

tests/cmd/remap-rootfs/remap-rootfs.go

@@ -1,3 +1,8 @@
+// remap-rootfs is a command-line tool to remap the ownership of an OCI
+// bundle's rootfs to match the user namespace id-mapping of the bundle's
+// config.json.
+//
+// This tool is only intended to be used within runc's integration tests.
 package main
 
 import (