Improve package source code context display on error (#37655)

Spack displays package code context when it shouldn't (e.g., on `FetchError`s)
and doesn't display it when it should (e.g., when errors occur in builder classes.
The line attribution can sometimes be off by one, as well.

- [x] Display package context when errors occur in a subclass of `PackageBase`
- [x] Display package context when errors occur in a subclass of `BaseBuilder`
- [x] Do not display package context when errors occur in `PackageBase`,
      `BaseBuilder` or other core code that is not in a `package.py` file.
- [x] Fix off-by-one error for core code (don't subtract one from the line number *unless*
      it's in an actual `package.py` file.

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Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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