pigeonhole: change package versioning

pigeonhole: change package versioning

The current pigeonhole Makefile is more complex than it needs to be, with
too many unique variables and the resulting package version
is currently this one:
```
dovecot-pigeonhole_2.3.21.0.5.21-r1_aarch64_cortex-a53.ipk
```

and based on Repology [1], it looks like we are the only GNU/Linux
distribution, who includes dovecot version and pigeonhole version
together. We should not include the extra dovecot version, because
even project [2]  website does not do it except their tarball.

What we can do better is that we added and modify a little bit
EXTRA_DEPENDS, which ensures that pigenhole 0.5.21.1 will be used
for Dovecot 2.3, because of that, we can have package version as
it should be.

```
dovecot-pigeonhole_0.5.21-r1_arm_cortex-a9_vfpv3-d16.ipk
```

Because of the changed versioning, we can remove the dynamic shell
execution for version extraction.

[1] https://repology.org/project/dovecot-pigeonhole/versions
[2] https://pigeonhole.dovecot.org/download

Fixes: 6c6a40ab57d1151f981237f81935b19486c45026 ("pigeonhole: fix runtime dependency on dovecot's ABI")
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>