Since #11598 sbang has been installed within the install_tree. This doesn’t play
nicely with install_tree padding, since sbang can’t do its job if it is installed in a
long path (this is the whole point of sbang).
This PR changes the padding specification. Instead of $padding inside paths,
we now have a separate `padding:` field in the `install_tree` configuration.
Previously, the `install_tree` looked like this:
```
/path/to/opt/spack_padding_padding_padding_padding_padding/
bin/
sbang
.spack-db/
...
linux-rhel7-x86_64/
...
```
```
This PR updates things to look like this:
/path/to/opt/
bin/
sbang
spack_padding_padding_padding_padding_padding/
.spack-db/
...
linux-rhel7-x86_64/
...
So padding is added at the start of all install prefixes *within* the unpadded
root. The database and all installations still go under the padded root.
This ensures that `sbang` is in the shorted possible path while also allowing
us to make long paths for relocatable binaries.
fafff0c6c0