formula: conditionally use executable name as completion name

[cho-m]

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In most cases, when running generate_completions_from_executable with a formula's bin/sbin executable, the resulting completion is usually intended for the executable itself.

From a quick glance, this should avoid majority of base_name overrides in Homebrew/core. Formulae that use register-python-argcomplete may still need an override (e.g. azure-cli, commitizen)

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For non-bin/sbin executables, the original default of formula name is used as these have higher chance of being an intermediary/helper executable, e.g. libexec/"bin/register-python-argcomplete" (azure-cli) or a helper go script (caddy)

[cho-m]

We could detect register-python-argcomplete and set default to commands.second. This does make the documentation a bit more complicated, but can provide a more useful default and avoid situations like current python-yq https://github.com/Homebrew/homebrew-core/blob/master/Formula/p/python-yq.rb#L49-L52

%w[yq xq tomlq].each do |script|
  generate_completions_from_executable(libexec/"bin/register-python-argcomplete", script,
                                       shell_parameter_format: :arg)
end

Above had unintended result of single set of completion named python-yq (each iteration overwriting same files).

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EDIT: However only 6 formulae use this command and 3 of them can use default of formula name. So may be simple enough to handle on Homebrew/core side.

Only need to fix python-yq: Homebrew/homebrew-core#201269

[MikeMcQuaid]

Makes sense to me, thanks!