Bugfix: cray manifest parsing regression (#37909)

fa7719a changed syntax for specifying exact versions, which are
required for some compiler specs (including those read as part
of parsing a Cray manifest). This fixes that and also makes a
couple other improvements to manifest parsing.

* Instantiate compiler specs with exact versions (fixes #37893)
* fix slingshot network detection (CPE 22.10+ has libcxi.so
  in /usr/lib64)
* "spack external find": add arg to ignore default dir for cray
  manifests

share/spack/spack-completion.bash

@@ -1060,7 +1060,7 @@ _spack_external_list() {
 }
 
 _spack_external_read_cray_manifest() {
-    SPACK_COMPREPLY="-h --help --file --directory --dry-run --fail-on-error"
+    SPACK_COMPREPLY="-h --help --file --directory --ignore-default-dir --dry-run --fail-on-error"
 }
 
 _spack_fetch() {