Summary:
This PR is step 1 to covering JIT'd methods and functions. Step 2 (using it in CI) is here: https://github.com/pytorch/pytorch/issues/56310.

1. This PR introduces a package `coverage_plugins` that hosts JITPlugin.
2. We also bring in a `.coveragerc` file that is used in CI to omit the files we don't want to report on (e.g., temporary directories or test or utils.)

**Disclaimer: This PR does NOT use the plug-in. Nothing should change as a result.**

Pull Request resolved: https://github.com/pytorch/pytorch/pull/56708

Test Plan:
CI. Coverage should not go down.

If you're interested in testing this plug-in locally, you should:
`pip install -e tools/coverage_plugins_package` from the root directory.
Add the following lines to `.coveragerc` under `[run]`
```
plugins =
    coverage_plugins.jit_plugin
```
And then try:
`coverage run test/test_jit.py TestAsync.test_async_script_no_script_mod`

You should see `.coverage.jit` show up at the end. You can then run `coverage combine --append` and `coverage debug data` to see that some files in `torch/jit` are covered.

Reviewed By: samestep

Differential Revision: D27945570

Pulled By: janeyx99

fbshipit-source-id: 78732940fcb498d5ec37d4075c4e7e08e96a8d55