* MM-16506: conditionally build go.mod for plugins
Unless `GO111MODULE=off`, generate a `go.mod` that points at the local copy of `mattermost-server` to ensure plugin tests that compile source code on demand always test with the local copy of mattermost-server. This also fixes an issue with early adopters of `GO111MODULE=on` on the server failing to find the right version of go-i18n dependencies.
* plugin: enable testlib resource management
* customize fileutils.Find* for testlib