* env depfile: allow deps only install
- Refactor `spack env depfile` to use a Jinja template, making it a bit
easier to follow as a human being.
- Add a layer of indirection in the generated Makefile through an
`<prefix>/.install-deps/<hash>` target, which allows one to specify
different options when installing dependencies. For example, only
verbose/debug mode on when installing some particular spec:
```
$ spack -e my_env env depfile -o Makefile --make-target-prefix example
$ make example/.install-deps/<hash> -j16
$ make example/.install/<hash> SPACK="spack -d" SPACK_INSTALL_FLAGS=--verbose -j16
```
This could be used to speed up `spack ci rebuild`:
- Parallel install of dependencies from buildcache
- Better readability of logs, e.g. reducing verbosity when installing
dependencies, and splitting logs into deps.log and current_spec.log
* Silence please!
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