move sbang to unpadded install tree root (#19640)

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.

etc/spack/defaults/config.yaml

@@ -20,6 +20,13 @@ config:
     root: $spack/opt/spack
     projections:
       all: "${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
+    # install_tree can include an optional padded length (int or boolean)
+    # default is False (do not pad)
+    # if padded_length is True, Spack will pad as close to the system max path
+    # length as possible
+    # if padded_length is an integer, Spack will pad to that many characters,
+    # assuming it is higher than the length of the install_tree root.
+    # padded_length: 128
 
 
   # Locations where templates should be found