bash-completion: instruct users to source proper file

[maxim-belkin]

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Currently, instructions in bash-completion (shouldn't it be bash-completions ???) formula are "unsafe". The right file to source is #{etc}/profile.d/bash_completion.sh. And here is why:

#{etc}/profile.d/bash_completion.sh file checks that:

• it is sourced from an interactive shell
• it has not been sourced already,
• Bash has the right version
• programmable completions are turned on
• #{etc}/bash_completion is readable
  and only then sources #{etc}/bash_completion

#{etc}/bash_completion, in turn, sources files #{etc}/bash_completion.d/* as necessary.

No changes to bottles are required.

[fxcoudert]

Thanks @maxim-belkin