Added support for querying by tags (#4786)

* Added support to query packages by tags.
    - The querying commands `spack list`, `spack find` and `spack info` have
      been modified to support querying by tags. Tests have been added to
      check that the feature is working correctly under what should be the
      most frequent use cases.

* Refactored Repo class to make insertion of new file caches easier.
    - Added the class FastPackageChecker. This class is a Mapping from
      package names to stat info, that gets memoized for faster access.

    - Extracted the creation of a ProviderIndex to its own factory function.

* Added a cache file for tags.

    - Following what was done for providers, a TagIndex class has been added.
      This class can serialize and deserialize objects from json. Repo and
      RepoPath have a new method 'packages_with_tags', that uses the TagIndex
      to compute a list of package names that have all the tags passed as
      arguments.

      On Ubuntu 14.04 the effect if the cache reduces the time for spack list
      from ~3sec. to ~0.3sec. after the cache has been built.

* Fixed colorization of `spack info`

var/spack/repos/builtin.mock/packages/mpich/package.py

@@ -31,6 +31,8 @@ class Mpich(Package):
     list_url   = "http://www.mpich.org/static/downloads/"
     list_depth = 2
 
+    tags = ['tag1', 'tag2']
+
     variant('debug', default=False,
             description="Compile MPICH with debug flags.")
 

var/spack/repos/builtin.mock/packages/mpich2/package.py

@@ -31,6 +31,8 @@ class Mpich2(Package):
     list_url   = "http://www.mpich.org/static/downloads/"
     list_depth = 2
 
+    tags = ['tag1', 'tag3']
+
     version('1.5', '9c5d5d4fe1e17dd12153f40bc5b6dbc0')
     version('1.4', 'foobarbaz')
     version('1.3', 'foobarbaz')

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

@@ -28,10 +28,9 @@
 
 class Aspa(MakefilePackage):
     """A fundamental premise in ExMatEx is that scale-bridging performed in
-        heterogeneous MPMD materials science simulations will place important
-        demands upon the exascale ecosystem that need to be identified and
-        quantified.
-        tags = proxy-app
+    heterogeneous MPMD materials science simulations will place important
+    demands upon the exascale ecosystem that need to be identified and
+    quantified.
     """
     tags = ['proxy-app']
     homepage = "http://www.exmatex.org/aspa.html"