The ASP based concretizer currently attaches non-declared dependencies on libc when trying to resolve for iconv. Doing a bisect the bug started at 0ce0864

Steps to reproduce the issue

At commit 0ce0864 or later:

$ spack solve diffutils
==> Best of 0 answers.
==> Optimization: [0, 0, 10, 0, 1, 0, 0, -2, 0]
diffutils@3.7%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
    ^libc@1.0%gcc@10.1.0~iconv~rpc arch=linux-ubuntu18.04-broadwell
    ^libiconv@1.16%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell

while the expected result is:

$ spack solve diffutils
==> Best of 0 answers.
==> Optimization: [0, 0, 10, 0, 1, 0, 0, -1, 0]
diffutils@3.7%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
    ^libiconv@1.16%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell

Error Message

No error message, but the DAG has a node that is unexpected.

Information on your system

• Spack: 0.16.0-440-0ce08640e0
• Python: 3.8.5
• Platform: linux-ubuntu18.04-broadwell
• Concretizer: clingo

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