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The environment variable name used to be AWS_DEFAULT_PROFILE but the
CLI documentation now only mentions AWS_PROFILE.

https://docs.aws.amazon.com/cli/latest/userguide/cli-environment.html

It seems like the CLI was the only tool using AWS_DEFAULT_PROFILE, and
all the AWS SDKs used AWS_PROFILE, so they standardized on it.

https://onetechnical.wordpress.com/2016/10/07/the-curious-case-of-aws_default_profile/

Note: still left AWS_DEFAULT_PROFILE on the method to set the profile to
maintain backwards compatibility.

Close #7354

* speed up chruby plugin by eliminating(as much as possible) calls to brew

* speed up aws plugin by eliminating(as much as possible) calls to brew

Sometimes the output of ``homebrew --prefix`` and ``homebrew --prefix
awscli`` don't match, and the second is the correct call.

Check _aws_zsh_completer_path had exist or not

the mangled version installed into $(brew-prefix)/bin if using
homebrew installed awscli on OSX.

This was discussed here - https://github.com/Homebrew/homebrew/issues/30268

It also supports quickly switching AWS profiles defined in
~/.aws/config using the asp alias