flake8: no wildcards in core; only import * from spack in packages

There are now separate flake8 configs for core vs. packages:
- core has a smaller set of flake8 exceptions
- packages allow `from spack import *` and module globals
- Allows core to take advantage of static checking for undefined names
- Allows packages to keep using Spack tricks like `from spack import *`
  and dependencies setting globals for dependents.

.flake8

@@ -1,8 +1,8 @@
 # -*- conf -*-
-# flake8 settings for Spack.
+# flake8 settings for Spack core files.
 #
-# Below we describe which flake8 checks Spack ignores and what the
-# rationale is.
+# These exceptions ar for Spack core files. We're slightly more lenient
+# with packages.  See .flake8_packages for that.
 #
 # Let people line things up nicely:
 # - E129: visually indented line with same indent as next logical line
@@ -13,14 +13,9 @@
 # Let people use terse Python features:
 # - E731: lambda expressions
 #
-# Spack allows wildcard imports:
-# - F403: disable wildcard import
-#
 # These are required to get the package.py files to test clean:
-# - F405: `name` may be undefined, or undefined from star imports: `module`
-# - F821: undefined name `name` (needed for cmake, configure, etc.)
 # - F999: syntax error in doctest
 #
 [flake8]
-ignore = E129,E221,E241,E272,E731,F403,F405,F821,F999
+ignore = E129,E221,E241,E272,E731,F999
 max-line-length = 79