tests: add checks and tests for consistent version() arguments

- Previously, Spack didn't check the arguments you put in version()
  directives.

- So, you could do something like this, where there are arguments for a
  URL fetcher AND for a git fetcher:

  version('1.0', md5='abc123', git='https://foo.bar', commit='feda2343')

- Now, we check the arguments before constructing a fetcher, to ensure
  that each package has *only* arguments for a single type of fetcher.

- Also added `test_package_version_consistency()` to the `package_sanity`
  test, so that all builtin packages are required to have valid
  `version()` directives.

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