Properly detect fmt (#9223)
Summary:
We tried to use fmt_8 from nix package in https://github.com/facebook/hhvm/issues/9131, however it did not take effect because the target created by find_package is `fmt::fmt`, not `fmt`.
This PR properly detect `fmt::fmt`, and create a `fmt` as a proxy to `fmt::fmt`.
Pull Request resolved: https://github.com/facebook/hhvm/pull/9223
Test Plan:
Run GitHub Action. The following line should be included in the build log:
```
hhvm> -- Using system fmt: /nix/store/qcgg8plb11j9dcz4r22hsaps2qfq8bnm-fmt-8.1.1-dev/lib/cmake/fmt
```
See https://github.com/facebook/hhvm/actions/runs/
3110469220/jobs/
5041685489
Reviewed By: Wilfred
Differential Revision:
D39760260
Pulled By: Atry
fbshipit-source-id:
5fa9b7b50560499c6162b06720ba63e33460f9f9