[WIP] Bugfix: add package AST check to ensure full hash works

[scheibelp]

(UPDATE 12/12: I'm working on an alternative to this: see #13620 (comment))

Closes #13265
Fixes #13237

Some valid python syntax in package.py files does not work with full hashes (these hashes include the DAG details as well as a hash of the package contents, and are used for creating buildcache tarballs). This adjusts the one package which was using logic that did not work, and also adds a spack package-ast-check command that is called as part of the CI to make sure that future edits to packages work with Spec.full_hash

[scheibelp]

UPDATE 12/12: I'm working on an alternative that will allow the swig package.py to remain unchanged; the new approach will default to including any AST content if it cannot determine whether to filter it out (as occurs for swig).