shell support: make `which spack` output intelligible (#19256)

Zsh and newer versions of bash have a builtin `which` function that will
show you if a command is actually an alias or a function. For functions,
the entire function is printed, and our `spack()` function is quite long.
Instead of printing out all that, make the `spack()` function a wrapper
around `_spack_shell_wrapper()`, and include some no-ops in the
definition so that users can see where it was created and where Spack is
installed.

Here's what the new output looks like in zsh:

```console
$ which spack
spack () {
	: this is a shell function from: /Users/gamblin2/src/spack/share/spack/setup-env.sh
	: the real spack script is here: /Users/gamblin2/src/spack/bin/spack
	_spack "$@"
	return $?
}
```

Note that `:` is a no-op in Bourne shell; it just discards anything after
it on the line. We use it here to embed paths in the function definition
(as comments are stripped).

1 files changed, 17 insertions, 6 deletions

diff --git a/share/spack/setup-env.sh b/share/spack/setup-env.sh
index 9d984ae199..5761d16362 100755
--- a/share/spack/setup-env.sh
+++ b/share/spack/setup-env.sh
@@ -45,7 +45,8 @@ if [ -n "${_sp_initializing:-}" ]; then
 fi
 export _sp_initializing=true
 
-spack() {
+
+_spack_shell_wrapper() {
     # Store LD_LIBRARY_PATH variables from spack shell function
     # This is necessary because MacOS System Integrity Protection clears
     # variables that affect dyld on process start.
@@ -243,11 +244,6 @@ _spack_determine_shell() {
 _sp_shell=$(_spack_determine_shell)
 
 
-# Export spack function so it is available in subshells (only works with bash)
-if [ "$_sp_shell" = bash ]; then
-    export -f spack
-fi
-
 alias spacktivate="spack env activate"
 
 #
@@ -315,6 +311,21 @@ if ! _spack_fn_exists use && ! _spack_fn_exists module; then
 	need_module="yes"
 fi;
 
+# Define the spack shell function with some informative no-ops, so when users
+# run `which spack`, they see the path to spack and where the function is from.
+eval "spack() {
+    : this is a shell function from: $_sp_share_dir/setup-env.sh
+    : the real spack script is here: $_sp_prefix/bin/spack
+    _spack_shell_wrapper \"\$@\"
+    return \$?
+}"
+
+# Export spack function so it is available in subshells (only works with bash)
+if [ "$_sp_shell" = bash ]; then
+    export -f spack
+    export -f _spack_shell_wrapper
+fi
+
 #
 # make available environment-modules
 #