[SPARK-46559][MLLIB] Wrap the export in the package name with backticks

[LuciferYang]

What changes were proposed in this pull request?

This pr wrap the export in the package name with backticks due to export will become keywords in Scala 3.

Why are the changes needed?

export will become keywords in Scala 3, Scala 2.13 compiler will check for relevant cases in the code, but it does not check for cases in the package name. However, if we write a Scala file as follows and compile it with Scala 3,

package org.apache.spark.mllib.pmml.export

private class ExportABC() {}

it will throw an error:

bin/scalac test.scala -explain                                                
-- [E040] Syntax Error: test.scala:1:36 ----------------------------------------
1 |package org.apache.spark.mllib.pmml.export
  |                                    ^^^^^^
  |                                an identifier expected, but 'export' found
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | If you want to use 'export' as identifier, you may put it in backticks: `export`.
   -----------------------------------------------------------------------------
1 error found

We can workaround by wrapping export with backticks.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Pass GitHub Actions

Was this patch authored or co-authored using generative AI tooling?

No

[LuciferYang]

cc @zhengruifeng This is a slightly ahead-of-time PR. I found that the modification did not break the mima check, so I fixed this issue on the fly. We can also wait until the Scala 2.13 compiler starts to throw errors or starts to support Scala 3 before fixing this issue.

[zhengruifeng]

also cc @WeichenXu123

[LuciferYang]

Merged into master. Thanks @zhengruifeng @HyukjinKwon