feat: abbreviated dependency tree printing

[mosteo]

This declutters the output of --tree when dependencies start to repeat by replacing repeats with ..., e.g.:

$ alr show --tree libgpr2
Dependencies (tree):
   gnat=14.1.3 (gnat_native) (>=14)
   gnatcoll=25.0.0 (~25.0.0)
   ├── gnat=14.1.3 (gnat_native) (>=13)
   └── libgpr=25.0.0 (~25.0.0)
       ├── gnat=14.1.3 (gnat_native) (/=2020)
       └── xmlada=25.0.0 (~25.0.0)
           └── gnat=14.1.3 (gnat_native) (>=11)
   gnatcoll_gmp=25.0.0 (~25.0.0)
   ├── gnatcoll=25.0.0 (~25.0.0) ···
   └── libgmp=6.3.0 (*)
   gnatcoll_iconv=25.0.0 (~25.0.0)
   └── gnatcoll=25.0.0 (~25.0.0) ···

vs (previous default)

$ alr -v show --tree libgpr2
Dependencies (tree):                                                       
   gnat=14.1.3 (gnat_native) (>=14)
   gnatcoll=25.0.0 (~25.0.0)
   ├── gnat=14.1.3 (gnat_native) (>=13)
   └── libgpr=25.0.0 (~25.0.0)
       ├── gnat=14.1.3 (gnat_native) (/=2020)
       └── xmlada=25.0.0 (~25.0.0)
           └── gnat=14.1.3 (gnat_native) (>=11)
   gnatcoll_gmp=25.0.0 (~25.0.0)
   ├── gnatcoll=25.0.0 (~25.0.0)
   │   ├── gnat=14.1.3 (gnat_native) (>=13)
   │   └── libgpr=25.0.0 (~25.0.0)
   │       ├── gnat=14.1.3 (gnat_native) (/=2020)
   │       └── xmlada=25.0.0 (~25.0.0)
   │           └── gnat=14.1.3 (gnat_native) (>=11)
   └── libgmp=6.3.0 (*)
   gnatcoll_iconv=25.0.0 (~25.0.0)
   └── gnatcoll=25.0.0 (~25.0.0)
       ├── gnat=14.1.3 (gnat_native) (>=13)
       └── libgpr=25.0.0 (~25.0.0)
           ├── gnat=14.1.3 (gnat_native) (/=2020)
           └── xmlada=25.0.0 (~25.0.0)
               └── gnat=14.1.3 (gnat_native) (>=11)

PR creation checklist

• A test is included, if required by the changes.
• doc/user-changes.md has been updated, if applicable.

[Fabien-Chouteau]

I would suggest a small change. Make the suspension points on a new line like so:

   gnatcoll_gmp=25.0.0 (~25.0.0)
   ├── gnatcoll=25.0.0 (~25.0.0) 
       └── ...

This better shows that there are dependencies omitted in the output in my opinion.

[mosteo]

Fair point.

[mosteo]

Now the output is:

Dependencies (tree):                                                       
   gnat=14.1.3 (gnat_native) (>=14)
   gnatcoll=25.0.0 (~25.0.0)
   ├── gnat=14.1.3 (gnat_native) (>=13)
   └── libgpr=25.0.0 (~25.0.0)
       ├── gnat=14.1.3 (gnat_native) (/=2020)
       └── xmlada=25.0.0 (~25.0.0)
           └── gnat=14.1.3 (gnat_native) (>=11)
   gnatcoll_gmp=25.0.0 (~25.0.0)
   ├── gnatcoll=25.0.0 (~25.0.0)
   │   └── ...
   └── libgmp=6.3.0 (*)
   gnatcoll_iconv=25.0.0 (~25.0.0)
   └── gnatcoll=25.0.0 (~25.0.0)
       └── ...