explicitly set the zlib path for libpng configure.
fixes:
```
[ ... ]
92 checking for memset... yes
93 checking for pow... no
94 checking for pow in -lm... yes
95 checking for clock_gettime... yes
96 checking for zlibVersion in -lz... no
97 checking for z_zlibVersion in -lz... no
>> 98 configure: error: zlib not installed
```
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
