package: Add fetch_options variable (#15317)

PR #15212 added a new connect_timeout option that can be overridden
using fetch_options but had to specified per-version. This adds a new
per-package variable that can be used to override fetch_options for
all versions in the package. This includes connect_timeout as well
as 'cookie' (e.g. for the jdk package).

Packages can combine package-level fetch_options with per-version
fetch_options, in which case the version fetch_options completely
override the package-level fetch_options.

This commit includes tests for the added behavior.

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

@@ -17,8 +17,7 @@ class Libffi(AutotoolsPackage):
     fetch_options = {'timeout': 60}
 
     version('3.2.1', sha256='d06ebb8e1d9a22d19e38d63fdb83954253f39bedc5d46232a05645685722ca37',
-            url="https://sourceware.org/pub/libffi/libffi-3.2.1.tar.gz",
-            fetch_options=fetch_options)
+            url="https://sourceware.org/pub/libffi/libffi-3.2.1.tar.gz")
 
     @property
     def headers(self):