The default siglevel is optional.

When the pacman sync db cannot be found include the missing database
which was registered.

Add a new option to specify an alternative database path which can be
used later for unit tests once there is a program to generate pacman db
files with specific build orders we want to test.

Add a new parameter which accepts a filename and outputs the dot
information to that file.

Use the petgraph library to create a DiGraph of the reverse dependency's.

For rebuilding we are interested in pkgbases instead of pkgnames
otherwise we get duplicate information.

Support multiple pkgnames as input and add at least one value as
requirement.

We always want to add reverse dependencies even if we already vistted
the node. A reproducer is cargo run haskell-basic-prelude and taskell
would end up not being included.

The previous approach would figure out the reverse dependencies of a
package by iterating over all packages for finding the reverse
dependencies of a package. Now we create one hashmap with the package to
reverse dependencies mapping which reduces the amount of work we have to
do and makes the program more or less constant time.