Improvements in the remotes management and API

[memsharded]

• Removing external methods related to removes, centralized in RemotesAPI
• Dropping global state ConanApp.selected_remotes and ConanApp.enabled_remotes
• Leaving legacy ConanApp.load_remotes() with only update things, to be cleaned in another PR, this is only about the remotes
• conan install will only use "enabled_remotes" those selected, so sources will not be retrieved from other repos not passed in -r=xxx arguments
• Order of arguments in conan install -r=remote1 -r=remote2 matters now