build-release: allow do build dpk or merged dpk+vm as a mod

We already have `build-release vm` command, it produced a `vm_0.dpk`
only containing the `.nexe` files.

Now we can do `build-release base` to build `unvanquished_0.dpk`
from `pkg/unvanquished_src.dpkdir`. It only contains built files
from the dpkdir.

We can also do `build-release mod` to buils `unvanquished_0.dpk`
with both files from `pkg/unvanquished_src.dpkdir` and the `.nexe`
files.

It uses Urcheon if available, otherwise it fallbacks on a script mimicking
what does Urcheon (but less complete and with no file conversion).
It may be good enough to build a mod dpk with a single command.

For debugging purposes the `--fake-urcheon` option makes possible
to use the script mimicking Urcheon even if Urcheon is available.

As usual the build folders are stored in `build/target` and the
produced files are stored in `build/release`.

The `--reference <ref>` option can be used to build a delta pak
from the given reference (requires Urcheon).

Author: illwieckz