`spack.repo.get()` can only be called on concrete specs (#31411)

The goal of this PR is to make clearer where we need a package object in Spack as opposed to a package class.

We currently instantiate a lot of package objects when we could make do with a class.  We should use the class
when we only need metadata, and we should only instantiate and us an instance of `PackageBase` at build time.

Modifications:
- [x] Remove the `spack.repo.get` convenience function (which was used in many places, and not really needed)
- [x] Use `spack.repo.path.get_pkg_class` wherever possible
- [x] Try to route most of the need for `spack.repo.path.get` through `Spec.package`
- [x] Introduce a non-data descriptor, that can be used as a decorator, to have "class level properties"
- [x] Refactor unit tests that had to be modified to reduce code duplication
- [x] `Spec.package` and `Repo.get` now require a concrete spec as input
- [x] Remove `RepoPath.all_packages` and `Repo.all_packages`

1 files changed, 3 insertions, 2 deletions

diff --git a/var/spack/repos/builtin/packages/gcc/package.py b/var/spack/repos/builtin/packages/gcc/package.py
index fe621a5bde..392f3b5910 100644
--- a/var/spack/repos/builtin/packages/gcc/package.py
+++ b/var/spack/repos/builtin/packages/gcc/package.py
@@ -11,6 +11,7 @@ import sys
 from archspec.cpu import UnsupportedMicroarchitecture
 
 import llnl.util.tty as tty
+from llnl.util.lang import classproperty
 
 import spack.platforms
 import spack.util.executable
@@ -351,8 +352,8 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
 
     build_directory = 'spack-build'
 
-    @property
-    def executables(self):
+    @classproperty
+    def executables(cls):
         names = [r'gcc', r'[^\w]?g\+\+', r'gfortran']
         suffixes = [r'', r'-mp-\d+\.\d', r'-\d+\.\d', r'-\d+', r'\d\d']
         return [r''.join(x) for x in itertools.product(names, suffixes)]