new command: `spack pkg grep` to search package files (#34388)

It's very common for us to tell users to grep through the existing Spack packages to
find examples of what they want, and it's also very common for package developers to do
it. Now, searching packages is even easier.

`spack pkg grep` runs grep on all `package.py` files in repos known to Spack. It has no
special options other than the search string; all options passed to it are forwarded
along to `grep`.

```console
> spack pkg grep --help
usage: spack pkg grep [--help] ...

positional arguments:
  grep_args  arguments for grep

options:
  --help     show this help message and exit
```

```console
> spack pkg grep CMakePackage | head -3
/Users/gamblin2/src/spack/var/spack/repos/builtin/packages/3dtk/package.py:class _3dtk(CMakePackage):
/Users/gamblin2/src/spack/var/spack/repos/builtin/packages/abseil-cpp/package.py:class AbseilCpp(CMakePackage):
/Users/gamblin2/src/spack/var/spack/repos/builtin/packages/accfft/package.py:class Accfft(CMakePackage, CudaPackage):
```

```console
> spack pkg grep -Eho '(\S*)\(PythonPackage\)' | head -3
AwsParallelcluster(PythonPackage)
Awscli(PythonPackage)
Bueno(PythonPackage)
```

## Return Value

This retains the return value semantics of `grep`:
* 0  for found,
* 1  for not found
* >1 for error

## Choosing a `grep`

You can set the ``SPACK_GREP`` environment variable to choose the ``grep``
executable this command should use.

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