* Better error message for spack providers
fixes#1355
`spack providers` now outputs a sensible error message if non-virtual
specs are provided as arguments:
```
$ spack providers mpi zlib petsc
==> Error: non-virtual specs cannot be part of the query [zlib, petsc]
```
Formatting of the output changed slightly.
* Calling 'spack providers' without arguments print the virtual pkg list
Also, the error message in case of a wrong parameter has been improved
to show the list of valid packages.
* Avoid printing headers if stdout is not a tty
* The provider list is formatted with colify if not in a tty
* Added a test to check the list of providers returned from the command
4bf7ce7d99