add alias hook for word "systemtap" pointing to systemtap package

Thanks for your bugreport.

This would require something like a "suggestion" feature. Just adding systemtap as a binary
to the database would not be enough. Otherwise the user installs systemtap, but still has
no binary systemtap and needs to guess what the name is. One solution is to provide a
symlink in the systemtap package itself, then it will be automatically picked up by c-n-f.

The other is something like:

molsson@molsson:~$ systemtap
Systemtap is provided by the systemtap package and the binary is called "stap".
bash: systemtap: command not found

Yet another solution would be to look not only at binaries but also at package names.
Something like:

molsson@molsson:~$ systemtap
There is no binary systemtap, but a package called systemtap that contains the
following binaries: "stap"

The attached patch implemens solution three:

$ ./command-not-found systemtap
No command 'systemtap' found.
But there is a package 'systemtap' that contains the binaries:
stap

Now the problem with that is that it needs to read all of the database (because the database
maps binary->package and provides no reverse mapping).

Now it appears that this does not make a difference (at least on my relatively fast system):

# echo 3 > /proc/sys/vm/drop_caches
# time ./command-not-found systemtap
No command 'systemtap' found.
But there is a package 'systemtap' that contains the binaries:
stap

real 0m3.330s
user 0m0.416s
sys 0m0.156s

# echo 3 > /proc/sys/vm/drop_caches
# time command-not-found systemtap
bash: command-not-found: command not found

real 0m3.350s
user 0m0.324s
sys 0m0.160s