Merge bitcoin/bitcoin#28507: contrib/bash-completions: use package naming conventions

360b917 contrib/bash-completions: use package naming conventions (Erik Arvstedt)

Pull request description:

  #### Copy of commit msg

  This naming scheme supports auto-detection and on-demand loading of completions.

  See
  https://github.com/scop/bash-completion/blob/ba109693ee2284f6a82f8f0e1563baf071252df9/README.md#faq,
  section "Where should I put it to be sure that interactive bash shells will find it and source it", keyword `foo.bash`.

  Previously, distro package maintainers had to rename these files manually.

ACKs for top commit:
  willcl-ark:
    ACK 360b917

Tree-SHA512: 6dd1f62309e877402fac2f7aec785d3053a1fd3fdeae38abc961c3f61269eb664a2489b6fa8294bd706d8ac387c7af3ac66cd7a852810f69371fa8ae991afacb

Author: fanquake