Git Ref versions can be paired to defined versions in the spec (#30998)

The current use of git ref's as a version requires a search algorithm to pick the right matching version based on the tags in the git history of the package.

This is less than ideal for the use case where users already know the specific version they want the git ref to be associated with. This PR makes a new version syntax [package]@[ref]=[version] to allow the users to specify the exact hash they wish to use.

var/spack/repos/builtin.mock/packages/depends-on-develop/package.py

@@ -0,0 +1,17 @@
+# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+from spack.package import *
+
+
+class DependsOnDevelop(Package):
+    homepage = "example.com"
+    url = "fake.com"
+
+    version("main", branch="main")
+    version("0.0.0", sha256="0123456789abcdef0123456789abcdef")
+
+    depends_on("develop-branch-version@develop")

var/spack/repos/builtin.mock/packages/develop-branch-version/package.py

@@ -0,0 +1,17 @@
+# Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack.package import *
+
+
+class DevelopBranchVersion(Package):
+    """Dummy package with develop version"""
+
+    homepage = "http://www.openblas.net"
+    url = "http://github.com/xianyi/OpenBLAS/archive/v0.2.15.tar.gz"
+    git = "https://github.com/dummy/repo.git"
+
+    version("develop", branch="develop")
+    version("0.2.15", "b1190f3d3471685f17cfd1ec1d252ac9")