Related to #9445: concretizing an older nonexistent version is causing an xsdk version to be accepted even though the xsdk tags are not related to the actual version number.

Steps to reproduce the issue

$ spack spec superlu-dist@:4.3
Input spec
--------------------------------
superlu-dist@:4.3

Concretized
--------------------------------
superlu-dist@xsdk-0.2.0 <snip...>

Information on your system

$ spack debug report
* **Spack:** 0.14.2-1466-32e5480ae
* **Python:** 3.7.6
* **Platform:** linux-rhel7-skylake_avx512

Additional information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output