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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