UI: Color external packages differently from installed packages (#49751)

Currently, externals show up in `spack find` and `spack spec` install status as a green
`[e]`, which is hard to distinguish from the green [+] used for installed packages.

- [x] Make externals magenta instead, so they stand out.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/spack/spec.py

@@ -206,7 +206,7 @@ class InstallStatus(enum.Enum):
 
     installed = "@g{[+]}  "
     upstream = "@g{[^]}  "
-    external = "@g{[e]}  "
+    external = "@M{[e]}  "
     absent = "@K{ - }  "
     missing = "@r{[-]}  "