Consolidate core_integration/datasource and rename parquet_source --> parquet_integration (#18226)

## Which issue does this PR close?
- related to #11210


## Rationale for this change

Running the following command

```shell
cargo test --test csv_schema_fix_test
```

Results in a 201 mb binary (and requires linking, etc) for only a few
tests

```shell
du -s -h target/debug/deps/csv_schema_fix_test-48507d40c6598e82
201M	target/debug/deps/csv_schema_fix_test-48507d40c6598e82
```


Let's combine that into the existing core integration test to save CI
runner space and time

Also, I have been confused why the parquet integration tests are named
`parquet_source` (I think it is meant to mirror `ParquetSource` which is
confusing) so let's change that to something more discoverable

## What changes are included in this PR?
1. Consolidate core_integration/datasource
2. rename `parquet_source` --> `parquet_integration`

## Are these changes tested?

It is only tests. You can run the tests in the new location via
```shell
cargo test --test core_integration -- csv
```

## Are there any user-facing changes?
No, this is all internal test rearrangement

Author: alamb

datafusion/core/tests/core_integration.rs

@@ -21,6 +21,9 @@ mod sql;
 /// Run all tests that are found in the `dataframe` directory
 mod dataframe;
 
+/// Run all tests that are found in the `datasource` directory
+mod datasource;
+
 /// Run all tests that are found in the `macro_hygiene` directory
 mod macro_hygiene;
 

…fusion/core/tests/csv_schema_fix_test.rs datafusion/core/tests/datasource/csv.rsdatafusion/core/tests/csv_schema_fix_test.rs renamed to datafusion/core/tests/datasource/csv.rs datafusion/core/tests/csv_schema_fix_test.rs renamed to datafusion/core/tests/datasource/csv.rs

@@ -0,0 +1,23 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Tests for various DataSources
+//!
+//! Note tests for the Parquet format are in `parquet_integration` binary
+
+// Include tests in csv module
+mod csv;