concretize.lp: drop 0 weight of external providers (#45025)

If an external happens to be a provider of anything, the solver would
set its weight to 0, meaning that it is most preferred, even if
packages.yaml config disagrees.

That was done so that `spack external find mpich` would be sufficent to
pick it up as mpi provider.

That may have made sense for mpi specifically, but doesn't make sense
for other virtuals. For example `glibc` provides iconv, and is an
external by design, but it's better to use libiconv as a separate
package as a provider.

Therefore, drop this rule, and instead let users add config:

```
mpi:
  require: [mpich]
```

or

```
mpi:
  buildable: false
```

which is well-documented.

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