Avoid stat-ing all packages at startup. (#7587)

- FastPackageChecker was being called at startup every time Spack runs,
  which takes a long time on networked filesystems.  Startup was taking
  5-7 seconds due to this call.

- The checker was intended to avaoid importing all packages (which is
  really expensive) when all it needs is to stat them.  So it's only
  "fast" for parts of the code that *need* it.

- This commit makes repositories instantiate the checker lazily, so it's
  only constructed when needed.

lib/spack/spack/repository.py

@@ -683,7 +683,7 @@ def check(condition, msg):
         self._instances = {}
 
         # Maps that goes from package name to corresponding file stat
-        self._fast_package_checker = FastPackageChecker(self.packages_path)
+        self._fast_package_checker = None
 
         # Index of virtual dependencies, computed lazily
         self._provider_index = None
@@ -928,9 +928,15 @@ def filename_for_package_name(self, spec):
         pkg_dir = self.dirname_for_package_name(spec.name)
         return join_path(pkg_dir, package_file_name)
 
+    @property
+    def _pkg_checker(self):
+        if self._fast_package_checker is None:
+            self._fast_package_checker = FastPackageChecker(self.packages_path)
+        return self._fast_package_checker
+
     def all_package_names(self):
         """Returns a sorted list of all package names in the Repo."""
-        return sorted(self._fast_package_checker.keys())
+        return sorted(self._pkg_checker.keys())
 
     def packages_with_tags(self, *tags):
         v = set(self.all_package_names())
@@ -952,7 +958,7 @@ def all_packages(self):
 
     def exists(self, pkg_name):
         """Whether a package with the supplied name exists."""
-        return pkg_name in self._fast_package_checker
+        return pkg_name in self._pkg_checker
 
     def is_virtual(self, pkg_name):
         """True if the package with this name is virtual, False otherwise."""