spack external find: show backtrace on error when --backtrace

[haampie]

Sometimes there are bugs in the implementation of a package's detection methods. Currently that's hard to troubleshoot:

$ spack [--backtrace] -e . external find python
==> Warning: error detecting "python" from prefix /usr/bin ['example']
==> No new external packages detected

With this PR you at least get a backtrace (also with --debug):

$ spack -e . external find python
==> Warning: error detecting "python" from prefix /usr/bin: [KeyError: 'example']

$ spack --backtrace -e . external find python
==> Warning: error detecting "python" from prefix /usr/bin: Traceback (most recent call last):
  File "/home/harmen/spack/lib/spack/spack/detection/path.py", line 275, in detect_specs
    pkg.determine_spec_details(candidate_path, items_in_prefix)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/harmen/spack/lib/spack/spack/package_base.py", line 241, in determine_spec_details
    variants = cls.determine_variants(objs, version_str)
  File "/home/harmen/spack/var/spack/repos/builtin/packages/python/package.py", line 386, in determine_variants
KeyError: 'example'

The line is still off by one, but hopefully one day it'll correct itself ;)