bugfix: no infinite recursion in setup-env.sh on Cray

On Cray platforms, we rely heavily on the module system to figure out
what targets, compilers, etc. are available. This unfortunately means
that we shell out to the `module` command as part of platform
initialization.

Because we run subcommands in a shell, we can get infinite recursion if
`setup-env.sh` and friends are in some init script like `.bashrc`.

This fixes the infinite loop by adding guards around `setup-env.sh`,
`setup-env.csh`, and `setup-env.fish`, to prevent recursive
initializations of Spack. This is safe because Spack never shells out to
itself, so we do not need it to be initialized in subshells.

- [x] add recursion guard around `setup-env.sh`
- [x] add recursion guard around `setup-env.csh`

share/spack/setup-env.csh

@@ -12,6 +12,13 @@
 #    setenv SPACK_ROOT /path/to/spack
 #    source $SPACK_ROOT/share/spack/setup-env.csh
 #
+
+# prevent infinite recursion when spack shells out (e.g., on cray for modules)
+if ($?_sp_initializing) then
+    exit 0
+endif
+setenv _sp_initializing true
+
 if ($?SPACK_ROOT) then
     set _spack_source_file = $SPACK_ROOT/share/spack/setup-env.csh
     set _spack_share_dir   = $SPACK_ROOT/share/spack
@@ -37,3 +44,6 @@ else
     echo "ERROR: Sourcing spack setup-env.csh requires setting SPACK_ROOT to "
     echo "       the root of your spack installation."
 endif
+
+# done: unset sentinel variable as we're no longer initializing
+unsetenv _sp_initializing

share/spack/setup-env.sh

@@ -39,6 +39,12 @@
 # spack module files.
 ########################################################################
 
+# prevent infinite recursion when spack shells out (e.g., on cray for modules)
+if [ -n "${_sp_initializing:-}" ]; then
+    exit 0
+fi
+export _sp_initializing=true
+
 spack() {
     # Store LD_LIBRARY_PATH variables from spack shell function
     # This is necessary because MacOS System Integrity Protection clears
@@ -358,3 +364,7 @@ _sp_multi_pathadd MODULEPATH "$_sp_tcl_roots"
 if [ "$_sp_shell" = bash ]; then
     source $_sp_share_dir/spack-completion.bash
 fi
+
+# done: unset sentinel variable as we're no longer initializing
+unset _sp_initializing
+export _sp_initializing