[MNG-6835] Building aggregate module using -pl option

**[Kypros Chrysanthou](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=chkypros)** opened **[MNG-6835](https://issues.apache.org/jira/browse/MNG-6835?redirect=false)** and commented

When building an aggregate module using the -pl option, its submodules are not picked up.

Using the -amd option, the submodules that are also child modules (i.e. they define the aggregate module as parent) are picked up.

I would expect that -am would "also make" the submodules but it seems that they are not identified as dependencies.

I have created a dummy project for showcasing/exploring this:

https://github.com/chkypros/aggregation

Acceptance Criteria:

- There needs to be a way to configure the build when using -pl to also build submodules.

- It should be possible to mirror the behaviour of "mvn \" using "mvn \ -pl . \"

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**Affects:** 3.6.3

Start by reproducing the aggregate-module case with the linked dummy project, comparing a normal mvn build with mvn -pl . and the -am/-amd options. Trace how -pl selects projects and how submodules are identified; done means -pl can be configured to include submodules and mirrors the normal build behavior.

Written by the indexing model from the issue text.