Composer has always allowed a single http_proxy (or CGI_HTTP_PROXY)
environment variable to be used for both HTTP and HTTPS requests. But
many other tools and libraries require scheme-specific values.
The landscape is already complicated by the use of and need for upper
and lower case values, so to bring matters inline with current practice
https_proxy is now required for HTTPS requests.
The new proxy handler incorporates a transition mechanism, which allows
http_proxy to be used for all requests when https_proxy is not set and
provides a `needsTransitionWarning` method for the main application.
Moving to scheme-specific environment variables means that a user may
set a single proxy for either HTTP or HTTPS requests. To accomodate this
situation during the transition period, an https_proxy value can be set
to an empty string which will prevent http_proxy being used for HTTPS
requests.
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