[MGPG-93] .asc files are written to a non-existent output directory

**[Pawel Veselov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=pveselov)** opened **[MGPG-93](https://issues.apache.org/jira/browse/MGPG-93?redirect=false)** and commented

I think this is an instance of MGPG-44, but for a different assembly.
Attempting to sign .war files created in non-standard location causes build to fail:

```
$ mvn package gpg:sign
[INFO] --- maven-gpg-plugin:3.0.1:sign (default-cli) @ b ---
gpg: using "856916A457583513" as default secret key for signing
gpg: can't create '/home/vps/tmp/gpg-test/mod/target/gpg/../artifacts/b-1-SNAPSHOT.war.asc': No such file or directory
gpg: signing failed: No such file or directory
```

Structure to reproduce: [^gpg-test.tar.gz]

This can be worked around with using `ascDirectory`, but I figured it should work without it.

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**Affects:** 3.0.1

**Attachments:**
- [gpg-test.tar.gz](https://issues.apache.org/jira/secure/attachment/13053931/gpg-test.tar.gz) (_820 bytes_)

Unpack gpg-test.tar.gz and reproduce the failure with `mvn package gpg:sign` against the non-standard WAR location. Trace how the signing output path is selected, including the `ascDirectory` workaround; done means the WAR signs successfully without requiring `ascDirectory`.

Written by the indexing model from the issue text.