[MSHADE-286] Shading fails when a dependency's main artifact does not exist

**[Peter De Maeyer](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=peterdm)** opened **[MSHADE-286](https://issues.apache.org/jira/browse/MSHADE-286?redirect=false)** and commented

Shading fails when a dependency's main artifact does not exist, see the methods `ShadeMojo.invalidMainArtifact/createErrorOutput` and their caller.
A similar existence check (luckily) does not exist for the other artifacts: test jar, sources, and test sources.
This was done intentionally, but it's overly strict because it prohibits a legitimate use case: some projects don't produce a main artifact, but only e.g. a test artifact.
Such projects can't be shaded because of this existence check.

It would be better to:
- Get rid of this check, or at least relax it, such that shading also works for projects that don't produce a main artifact.
- Complete the symmetry between jar, test jar, sources and test sources by adding a `shadeJar` boolean with default value `true`, which disables shading of main artifacts in a similar way `shadeTestJar`, `createSourcesJar` and `createTestSourcesJar` work. This will allow shading to disable creation of a main artifact altogether, even when the dependencies _do_ have a main artifact.

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**Affects:** 3.1.0

**Issue Links:**
- [MSHADE-284](https://issues.apache.org/jira/browse/MSHADE-284) Shaded test JARs are always empty

Start in ShadeMojo at invalidMainArtifact and createErrorOutput, then inspect their caller and the existing shadeTestJar, createSourcesJar, and createTestSourcesJar handling. Determine whether the main-artifact check should be removed or controlled by a shadeJar option; done means dependencies without a main artifact can be shaded without this failure and main-artifact shading can be disabled consistently.

Written by the indexing model from the issue text.