spack uninstall fails for libxml2

[lee218llnl]

spack uninstall fails for libxml2, when without python bindings.

cab687@lee218:spack uninstall libxml2
Traceback (most recent call last):
File "/nfs/tmp2/lee218/pkg/spack3/spack/bin/spack", line 163, in
main()
File "/nfs/tmp2/lee218/pkg/spack3/spack/bin/spack", line 144, in main
return_val = command(parser, args)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/cmd/uninstall.py", line 96, in uninstall
pkg.do_uninstall(force=args.force)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/package.py", line 932, in do_uninstall
spack.hooks.pre_uninstall(self)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/hooks/init.py", line 69, in call
hook(pkg)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/hooks/extensions.py", line 35, in pre_uninstall
if pkg.activated:
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/package.py", line 518, in activated
exts = spack.install_layout.extension_map(self.extendee_spec)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/directory_layout.py", line 345, in extension_map
_check_concrete(spec)
File "/nfs/tmp2/lee218/pkg/spack3/spack/lib/spack/spack/directory_layout.py", line 44, in _check_concrete
raise ValueError('Specs passed to a DirectoryLayout must be concrete!')
ValueError: Specs passed to a DirectoryLayout must be concrete!

[tgamblin]

@lee218llnl: does the fix I just checked in work for you? The database support needs to properly mark specs representing a real install as concrete, as well.