packages: Raise an exception for invalid 'license' values.

This is written in such a way that the type check turns into a no-op at
macro-expansion time for trivial cases:

  > ,optimize (validate-license gpl3+)
  $18 = gpl3+
  > ,optimize (validate-license (list gpl3+ gpl2+))
  $19 = (list gpl3+ gpl2+)

* guix/packages.scm (valid-license-value?, validate-license): New
macros.
(<package>)[license]: Add 'sanitize' option.
(&package-license-error): New error condition type.
* tests/packages.scm ("license type checking"): New test.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/packages.scm b/tests/packages.scm
index 6cbc34ba0b..dc03b13417 100644
--- a/tests/packages.scm
+++ b/tests/packages.scm
@@ -94,6 +94,13 @@
                     (write
                      (dummy-package "foo" (location #f)))))))
 
+(test-equal "license type checking"
+  'bad-license
+  (guard (c ((package-license-error? c)
+             (package-error-invalid-license c)))
+    (dummy-package "foo"
+      (license 'bad-license))))
+
 (test-assert "hidden-package"
   (and (hidden-package? (hidden-package (dummy-package "foo")))
        (not (hidden-package? (dummy-package "foo")))))