fix: source files using absolute paths for absolute BASH_SOURCE

Some completion functions use BASH_SOURCE to identify the path to the
file where the functions are defined.  However, if the file was
sourced with the relative path (e.g. `. ./completions/make`),
BASH_SOURCE referenced by the function contains the relative path.
This causes the problem after the current working directory is changed
from the one where the file was sourced.  To make BASH_SOURCE
available to the completion files, we should replace a relative path
to the absolute path before passing the path to `source` or `.`.

To supply the absolute path, we add a new global variable
`_comp__base_directory`, which contains the directory where
`bash_completion` is located.

bash_completion

@@ -3125,6 +3125,18 @@ _comp_complete_minimal()
 # https://lists.gnu.org/archive/html/bug-bash/2012-01/msg00045.html
 complete -F _comp_complete_minimal ''
 
+# Initialize the variable "_comp__base_directory"
+# @var[out] _comp__base_directory
+_comp__init_base_directory()
+{
+    local REPLY
+    _comp_abspath "${BASH_SOURCE[0]-./bash_completion}"
+    _comp__base_directory=${REPLY%/*}
+    [[ $_comp__base_directory ]] || _comp__base_directory=/
+    unset -f "$FUNCNAME"
+}
+_comp__init_base_directory
+
 # @since 2.12
 _comp_load()
 {
@@ -3177,11 +3189,7 @@ _comp_load()
     # we want to prefer in-tree completions over ones possibly coming with a
     # system installed bash-completion. (Due to usual install layouts, this
     # often hits the correct completions in system installations, too.)
-    if [[ $BASH_SOURCE == */* ]]; then
-        dirs+=("${BASH_SOURCE%/*}/completions")
-    else
-        dirs+=(./completions)
-    fi
+    dirs+=("$_comp__base_directory/completions")
 
     # 3) From bin directories extracted from the specified path to the command,
     # the real path to the command, and $PATH
@@ -3323,12 +3331,10 @@ _comp__init_collect_startup_configs()
         # run-in-place-from-git-clone setups.  Notably we do it after the
         # system location here, in order to prefer in-tree variables and
         # functions.
-        if [[ ${base_path%/*} == */share/bash-completion ]]; then
-            compat_dir=${base_path%/share/bash-completion/*}/etc/bash_completion.d
-        elif [[ $base_path == */* ]]; then
-            compat_dir="${base_path%/*}/bash_completion.d"
+        if [[ $_comp__base_directory == */share/bash-completion ]]; then
+            compat_dir=${_comp__base_directory%/share/bash-completion}/etc/bash_completion.d
         else
-            compat_dir=./bash_completion.d
+            compat_dir=$_comp__base_directory/bash_completion.d
         fi
         [[ ${compat_dirs[0]} == "$compat_dir" ]] ||
             compat_dirs+=("$compat_dir")