Fixes #681

There are many legitimate use-cases where mockery needs to include auto-generated files
in its recursive package discovery. We add an `include-auto-generated` parameter that
is set to `true` by default, but can be set to `false` if users want to retain the
old behavior. We are considering the old behavior to be a bug due to the large number
of GitHub issues we'll likely get. The original motivation for adding this behavior
was fixed through a different mechanism, which is why we're considering this to be
a bugfix.

Include auto-generated files in recursive discovery by default

This adds e2e tests for #681, which asserts packages containing only
auto-generated files are added to the list of importable packages
when doing recursive package discovery.

Add e2e tests for issue #681

This commit will log an error message if unsupported config is being used. I have chosen not to cause mockery to return an error code because it's possible that configs using unsupported parameters are working just fine, so we don't want to break those users. Instead, the log message should hopefully get their attention, and the attention of anyone trying to migrate to packages. Also removed is the conditional used to enter into the `packages` code path. Previously it asserted that `Config.Name == ""` but this was causing more confusion than it needed to.

This fixes #685

Add error log for unsupported config

Add `include-regex` option

Add gcc to docker for projects with cgo

Technically, the outputter should not be responsible for determining if the interface
should be generated. That is higher-order that should have been consulted before
calling `outputter.Generate`.

Minor refactor and dead code removal