eth-json-rpc-provider migration - B4: Add exceptions in `constraint…

…s.pro` for ISC license. (#1767)

## Explanation

This PR implements the following incremental step in the process for
migrating `eth-json-rpc-provider` into the core monorepo:

***

### Phase B: Staging in `migrated-packages/`
#### 4. Add exception for non-MIT license.
- [x] If migration target uses a non-MIT license, add exception entries
in the root `constraints.pro` file.
- [x] Make sure the new rule doesn't break any of the existing
package.json files by running `yarn constraints`.

***

See #1551 (comment)
for an outline of the entire process.

## References

- Contributes to #1685
- Contributes to #1551

## Changelog

N/A

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

constraints.pro

@@ -207,9 +207,14 @@ gen_enforced_field(WorkspaceCwd, 'repository.url', RepoUrl) :-
   repo_name(RepoUrl, _).
   WorkspaceCwd \= '.'.
 
-% The license for all published packages must be MIT.
+% The license for all published packages must be MIT unless otherwise specified.
 gen_enforced_field(WorkspaceCwd, 'license', 'MIT') :-
-  \+ workspace_field(WorkspaceCwd, 'private', true).
+  \+ workspace_field(WorkspaceCwd, 'private', true),
+  WorkspaceCwd \= 'packages/eth-json-rpc-provider'.
+% The following published packages use an ISC license instead of MIT.
+gen_enforced_field(WorkspaceCwd, 'license', 'ISC') :-
+  \+ workspace_field(WorkspaceCwd, 'private', true),
+  WorkspaceCwd == 'packages/eth-json-rpc-provider'.
 % Non-published packages do not have a license.
 gen_enforced_field(WorkspaceCwd, 'license', null) :-
   workspace_field(WorkspaceCwd, 'private', true).