provide main for brotli and non-brotli builds

[withinboredom]

Provides a brotli and non-brotli main file for caddy.

fixes #1377

[dunglas]

I don't think it's necessary.

The main.go file is just there for convenience for "internal" (or at least "standard" builds: they allow to be sure of the exact dependencies used.

Users should use xcaddy for custom builds with custom modules or dependencies. xcaddy will generate the appropriate main.go file.

[withinboredom]

Makes sense!