Running `spack module refresh -m lmod --delete-tree -y` throws the
following warning:
```console
==> Warning: Could not write module file [/Users/oxberry1/spack/share/spack/lmod/darwin-sierra-x86_64/openblas/0.2.20-ibhtl5q/clang/5.0.1/ipopt/3.12.9.lua]
==> Warning: --> global name 'prefix' is not defined <--
```
This warning arises because the variable expansions in
`spack.modules.BaseFileWriter` occur at module/package scope. Even
though `prefix` is an argument to `install` methods in e.g.,
`MakefilePackage`, so using this argument within package methods is
legal, doing so defeats the introspection logic in
`spack.modules.BaseFileWriter`. Replacing `prefix` with `self.prefix`
resolves the problem, because the introspection logic in
`spack.modules.BaseFileWriter` can use introspection to query IPOPT's
prefix.
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