BS-94/use internal sources

This is a new omnibus feature that allows users to build from only internal sources.

It works by adding a configuration value to the omnibus.rb file:

omnibus.rb

use_internal_sources true

And then in the software definitions, you can define an internal_source like this:

my-software/cacerts.rb

internal_source url: "https://my.internal.source/cacert-#{version}.pem"

When both source and internal_source are set, it will use the internal_source when use_internal_sources is set to true.

source url: "https://curl.se/ca/cacert-#{version}.pem"
internal_source url: "https://my.internal.source/cacert-#{version}.pem" # uses this one

When use_s3_caching is enabled, it will still use the cached sources wherever possible.
When use_internal_sources is enabled and no internal_source is set for a dependency with a url: it will throw and exception

Please ensure that you check for:

[ ] If this change impacts git cache validity, it bumps the git cache serial number
[ ] If this change impacts compatibility with omnibus-software, the corresponding change is reviewed and there is a release plan
[ ] If this change impacts compatibility with the omnibus cookbook, the corresponding change is reviewed and there is a release plan