Fix 1635 unexpected missing label error when ignoring header case

[amelie-rondot]

• fixes Unexpected "missing-label" error with option header_case = False  #1635

---

This PR fixes unexpected missing-label error in validation report which occurs when validating tabular data in this specific case: The data contains a named column corresponding to a required fieldin the schema used for validation, written without respecting case-sensitivity.
For example:

data = [["aa", "BB"], ["a", "b"]]
schema = {
        "$schema": "https://frictionlessdata.io/schemas/table-schema.json",
        "fields": [
            {"name": "AA", "constraints": {"required": True}},
            {"name": "bb", "constraints": {"required": True}}
        ]
    }

This PR adds some test cases:

• one test case to ensure that validating this tabular data with this schema and using schema_sync=True and dialect=Dialect(header_case=False) options, returns a valid report.
• one test case to ensure that if validation is case-sensitive, if a name column does not respect case a missing-label occurs in validation report.
• another test case to ensure that validating with missing required column "AA" with this schema and using schema_sync=True and dialect=Dialect(header_case=False) options, returns an invalid report with missing-label error related to field "AA".

Finally, I suggest some refactoring in this PR:

• refactoring removing missing required label from field info refactoring removing missing required label from field info
• refactoring creating schema fields among labels part when using schema_sync option in detect_schema() Detector method

[amelie-rondot]

This PR is replaced by PR#1641