Speed up concretization (#5716)

This isn't a rework of the concretizer but it speeds things up a LOT.

The main culprits were:
  1. Variant code, `provider_index`, and `concretize.py` were calling
     `spec.package` when they could use `spec.package_class`
    - `spec.package` looks up a package instance by `Spec`, which requires a
      (fast-ish but not that fast) DAG compare.
    - `spec.package_class` just looks up the package's class by name, and you
        should use this when all you need is metadata (most of the time).
    - not really clear that the current way packages are looked up is
      necessary -- we can consider refactoring that in the future.

  2. `Repository.repo_for_pkg` parses a `str` argument into a `Spec` when
     called with one, via `@_autospec`, but this is not needed.
     - Add some faster code to handle strings directly and avoid parsing

This speeds up concretization 3-9x in my limited tests.  Still not super
fast but much more bearable:

Before:
  - `spack spec xsdk` took 33.6s
  - `spack spec dealii` took 1m39s

After:
  - `spack spec xsdk` takes 6.8s
  - `spack spec dealii` takes 10.8s

4 files changed, 23 insertions, 17 deletions

diff --git a/lib/spack/spack/concretize.py b/lib/spack/spack/concretize.py
index 1aed7d8eb1..10a3705083 100644
--- a/lib/spack/spack/concretize.py
+++ b/lib/spack/spack/concretize.py
@@ -146,8 +146,8 @@ class DefaultConcretizer(object):
             return False
 
         # List of versions we could consider, in sorted order
-        pkg = spec.package
-        usable = [v for v in pkg.versions
+        pkg_versions = spec.package_class.versions
+        usable = [v for v in pkg_versions
                   if any(v.satisfies(sv) for sv in spec.versions)]
 
         yaml_prefs = PackagePrefs(spec.name, 'version')
@@ -165,7 +165,7 @@ class DefaultConcretizer(object):
             -yaml_prefs(v),
 
             # The preferred=True flag (packages or packages.yaml or both?)
-            pkg.versions.get(Version(v)).get('preferred', False),
+            pkg_versions.get(Version(v)).get('preferred', False),
 
             # ------- Regular case: use latest non-develop version by default.
             # Avoid @develop version, which would otherwise be the "largest"
diff --git a/lib/spack/spack/provider_index.py b/lib/spack/spack/provider_index.py
index 054f1ca807..15cb3e785e 100644
--- a/lib/spack/spack/provider_index.py
+++ b/lib/spack/spack/provider_index.py
@@ -97,8 +97,8 @@ class ProviderIndex(object):
 
         assert(not spec.virtual)
 
-        pkg = spec.package
-        for provided_spec, provider_specs in iteritems(pkg.provided):
+        pkg_provided = spec.package_class.provided
+        for provided_spec, provider_specs in iteritems(pkg_provided):
             for provider_spec in provider_specs:
                 # TODO: fix this comment.
                 # We want satisfaction other than flags
diff --git a/lib/spack/spack/repository.py b/lib/spack/spack/repository.py
index 0a43cf8af1..ba33e24069 100644
--- a/lib/spack/spack/repository.py
+++ b/lib/spack/spack/repository.py
@@ -537,20 +537,27 @@ class RepoPath(object):
         sys.modules[fullname] = module
         return module
 
-    @_autospec
     def repo_for_pkg(self, spec):
         """Given a spec, get the repository for its package."""
-        # If the spec already has a namespace, then return the
-        # corresponding repo if we know about it.
-        if spec.namespace:
-            fullspace = '%s.%s' % (self.super_namespace, spec.namespace)
-            if fullspace not in self.by_namespace:
-                raise UnknownNamespaceError(spec.namespace)
-            return self.by_namespace[fullspace]
+        # We don't @_autospec this function b/c it's called very frequently
+        # and we want to avoid parsing str's into Specs unnecessarily.
+        if isinstance(spec, spack.spec.Spec):
+            # If the spec already has a namespace, then return the
+            # corresponding repo if we know about it.
+            if spec.namespace:
+                fullspace = '%s.%s' % (self.super_namespace, spec.namespace)
+                if fullspace not in self.by_namespace:
+                    raise UnknownNamespaceError(spec.namespace)
+                return self.by_namespace[fullspace]
+            name = spec.name
+
+        else:
+            # handle strings directly for speed instead of @_autospec'ing
+            name = spec
 
         # If there's no namespace, search in the RepoPath.
         for repo in self.repos:
-            if spec.name in repo:
+            if name in repo:
                 return repo
 
         # If the package isn't in any repo, return the one with
diff --git a/lib/spack/spack/variant.py b/lib/spack/spack/variant.py
index 3f9ede1047..4553bf53e4 100644
--- a/lib/spack/spack/variant.py
+++ b/lib/spack/spack/variant.py
@@ -592,10 +592,9 @@ def substitute_abstract_variants(spec):
         spec: spec on which to operate the substitution
     """
     for name, v in spec.variants.items():
-        pkg_cls = type(spec.package)
-        pkg_variant = spec.package.variants[name]
+        pkg_variant = spec.package_class.variants[name]
         new_variant = pkg_variant.make_variant(v._original_value)
-        pkg_variant.validate_or_raise(new_variant, pkg_cls)
+        pkg_variant.validate_or_raise(new_variant, spec.package_class)
         spec.variants.substitute(new_variant)