Better error message for spack providers (#7748)

* 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

0 files changed, 0 insertions, 0 deletions