Speedup environment activation (#13557)

* Add a transaction around repeated calls to `spec.prefix` in the activation process
* cache the computation of home in the python package to speed up setting deps
* ensure that module-scope variables are only set *once* per module

var/spack/repos/builtin/packages/python/package.py

@@ -163,6 +163,9 @@ class Python(AutotoolsPackage):
     _DISTUTIL_CACHE_FILENAME = 'sysconfig.json'
     _distutil_vars = None
 
+    # Used to cache home locations, since computing them might be expensive
+    _homes = {}
+
     # An in-source build with --enable-optimizations fails for python@3.X
     build_directory = 'spack-build'
 
@@ -622,8 +625,11 @@ def home(self):
         ``packages.yaml`` unknowingly. Query the python executable to
         determine exactly where it is installed."""
 
-        prefix = self.get_config_var('prefix')
-        return Prefix(prefix)
+        dag_hash = self.spec.dag_hash()
+        if dag_hash not in self._homes:
+            prefix = self.get_config_var('prefix')
+            self._homes[dag_hash] = Prefix(prefix)
+        return self._homes[dag_hash]
 
     @property
     def libs(self):