helper/schema: InternalValidate fix for configuration block type checking [v1]

[bflad]

The validation for AtLeastOneOf, ConflictsWith, ExactlyOneOf, and RequiredWith should not flag configuration block attributes without Type: TypeList and MaxItems: 1 if they are the last part of the reference.

Previously:

    --- FAIL: TestSchemaMap_InternalValidate/RequiredWith_string_syntax_with_list_configuration_block (0.00s)
        schema_test.go:5037: "RequiredWith string syntax with list configuration block": Unexpected error occurred: RequiredWith: test configuration block reference (config_block_attr) can only be used with TypeList and MaxItems: 1 configuration blocks
    --- FAIL: TestSchemaMap_InternalValidate/AtLeastOneOf_string_syntax_with_set_configuration_block (0.00s)
        schema_test.go:5037: "AtLeastOneOf string syntax with set configuration block": Unexpected error occurred: AtLeastOneOf: test configuration block reference (config_block_attr) can only be used with TypeList and MaxItems: 1 configuration blocks
...

[ghost]

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