fix: parallel file validation for a datapackage (#1722)

- fixes #1644 

### Context

- In validator.py, `Resource` was imported as a type, but then used as a
class to retrieve a class method (`Resource.from_descriptor`).
- The tests would not intercept this error, as they were performed on a
schema with a foreign key, and the validation would fall back on
synchronous validation in this case.
- Importing `Resource` (not as a type) into `validator.py` leads to a
circular import. A small workaround would have been to import Resource
locally, but this was not very satisfactory.

### Suggested changes and fixes

- I repaired the tests, using datapackages with no foreign keys to test
parallel validation. I checked that these tests would have failed
because with the bug.
- I moved the implementation of `Validator.validate_resource` and
`Validator.validate_package` methods to the `Resource.validate` and
`Package.validate` methods. This solves the circular import problem, and
removes duplication (method signatures).
- The tests have been dispatched as well.
- I soft deprecated the `Validator` class, keeping the methods
(forwarding input parameters) as they are part of the public API. No
warning, only a deprecation message in the docstring, and a mention that
there is no plan to remove the class in the future.

### Note 

I could not produce any significant performance gain in my tests, I
wonder if there is some shared resources' lock that prevents the
validation from effectively running the validation in parallel (but I
have not spent much time on this yet).



<details> 
<summary>Archive of explorations</summary>

- [X] Test that it works correctly
- Works without error, however, I could not produce any significant
performance gain in my tests.
- [X] Check why the tests did not intercept this error
- The tests for parallel processing are run for a schema with a foreign
key - which fall backs to single-core processing. So the
`validate_parallel` function was actually never executed in the tests.
- [X] See if I can find a more elegant way to deal with the circular
import. I actually don't see the benefit of having this validator.py
class instead of dispatching the methods to `Resource` and `DataPackage`
directly, but I may have missed something.
</details>

Author: pierrecamilleri

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 Here described only the breaking and most significant changes. The full changelog and documentation for all released versions could be found in nicely formatted [commit history](https://github.com/frictionlessdata/frictionless-py/commits/main).
 
+## v5.18.1
+
+- feat: add pass_row as configurable parameter for field_update 
+  ([#1729](https://github.com/frictionlessdata/frictionless-py/pull/1729))
+- fix: vendor unmaintained stringcase library 
+  ([#1727](https://github.com/frictionlessdata/frictionless-py/pull/1727))
+- Various bug fixes
+
 ## v5.18
 
 - Support `ignore_constraints` option for the `Indexer` (#1691)

data/invalid/datapackage_no_foreign_key.json

@@ -0,0 +1,49 @@
+{
+    "name": "testing",
+    "resources": [
+        {
+            "name": "data",
+            "path": "data.csv",
+            "schema": {
+                "fields": [
+                    {
+                        "name": "id",
+                        "type": "string",
+                        "constraints": {
+                            "required": true
+                        }
+                    },
+                    {
+                        "name": "name",
+                        "type": "string"
+                    },
+                    {
+                        "name": "description",
+                        "type": "string"
+                    },
+                    {
+                        "name": "amount",
+                        "type": "number"
+                    }
+                ],
+                "primaryKey": "id"
+            }
+        },
+        {
+            "name": "data2",
+            "path": "data2.csv",
+            "schema": {
+                "fields": [
+                    {
+                        "type": "string",
+                        "name": "parent"
+                    },
+                    {
+                        "type": "string",
+                        "name": "comment"
+                    }
+                ]
+            }
+        }
+    ]
+}

frictionless/__init__.py

@@ -40,4 +40,6 @@
 from .table import Lookup as Lookup
 from .table import Row as Row
 from .transformer import Transformer as Transformer
+
+# Deprecated
 from .validator import Validator as Validator