Use JSON for the database instead of YAML. (#2189)

* Use JSON for the database instead of YAML.

- JSON is much faster than YAML *and* can preserve ordered keys.
  - 170x+ faster than Python YAML when using unordered dicts
  - 55x faster than Python YAML (both using OrderedDicts)
  - 8x faster than C YAML (with OrderedDicts)

- JSON is built into Python, unlike C YAML, so doesn't add a dependency.
- Don't need human readability for the package database.
- JSON requires no major changes to the code -- same object model as YAML.
- add to_json, from_json methods to spec.

* Add tests to ensure JSON and YAML don't need to be ordered in DB.

* Write index.json first time it's not found instead of requiring reindex.

* flake8 bug.

bin/spack

@@ -41,14 +41,6 @@ SPACK_PREFIX = os.path.dirname(os.path.dirname(SPACK_FILE))
 SPACK_LIB_PATH = os.path.join(SPACK_PREFIX, "lib", "spack")
 sys.path.insert(0, SPACK_LIB_PATH)
 
-# Try to use system YAML if it is available, as it might have libyaml
-# support (for faster loading via C).  Load it before anything in
-# lib/spack/external so it will take precedence over Spack's PyYAML.
-try:
-    import yaml
-except ImportError:
-    pass   # ignore and use slow yaml
-
 # Add external libs
 SPACK_EXTERNAL_LIBS = os.path.join(SPACK_LIB_PATH, "external")
 sys.path.insert(0, SPACK_EXTERNAL_LIBS)