contrib/bash-completions: use package naming conventions

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".

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

contrib/completions/bash/bitcoin-tx.bash

@@ -0,0 +1,57 @@
+# bash programmable completion for bitcoin-tx(1)
+# Copyright (c) 2016-2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+_bitcoin_tx() {
+    local cur prev words=() cword
+    local bitcoin_tx
+
+    # save and use original argument to invoke bitcoin-tx for -help
+    # it might not be in $PATH
+    bitcoin_tx="$1"
+
+    COMPREPLY=()
+    _get_comp_words_by_ref -n =: cur prev words cword
+
+    case "$cur" in
+        load=*:*)
+            cur="${cur#load=*:}"
+            _filedir
+            return 0
+            ;;
+        *=*)	# prevent attempts to complete other arguments
+            return 0
+            ;;
+    esac
+
+    if [[ "$cword" == 1 || ( "$prev" != "-create" && "$prev" == -* ) ]]; then
+        # only options (or an uncompletable hex-string) allowed
+        # parse bitcoin-tx -help for options
+        local helpopts
+        helpopts=$($bitcoin_tx -help | sed -e '/^  -/ p' -e d )
+        COMPREPLY=( $( compgen -W "$helpopts" -- "$cur" ) )
+    else
+        # only commands are allowed
+        # parse -help for commands
+        local helpcmds
+        helpcmds=$($bitcoin_tx -help | sed -e '1,/Commands:/d' -e 's/=.*/=/' -e '/^  [a-z]/ p' -e d )
+        COMPREPLY=( $( compgen -W "$helpcmds" -- "$cur" ) )
+    fi
+
+    # Prevent space if an argument is desired
+    if [[ $COMPREPLY == *= ]]; then
+        compopt -o nospace
+    fi
+
+    return 0
+} &&
+complete -F _bitcoin_tx bitcoin-tx
+
+# Local variables:
+# mode: shell-script
+# sh-basic-offset: 4
+# sh-indent-comment: t
+# indent-tabs-mode: nil
+# End:
+# ex: ts=4 sw=4 et filetype=sh