Allow defining complex expression like in interactive mode

[ggkochanski]

Overlapping parameter expression for goal help:evaluate by default:
handleResponse("${" + expression + "}", output);

limits usage for more complex expressions, e.g.
mvn help:evaluate -Dexpression='project_version=${project.version}'

which currently fails with message:
null object or invalid expression

Skipping the overlapping when an expression already contains string '${' removes this limitation and allows to define expression in the same way as in the interactive mode:
echo 'project_version=${project.version}'|mvn help:evaluate

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[slawekjaranowski]

@ggkochanski please add at least one line from template to PR description

[ ] I hereby declare this contribution to be licenced under the [Apache License Version 2.0, January 2004](http://www.apache.org/licenses/LICENSE-2.0)

https://github.com/apache/maven-help-plugin/blob/master/.github/pull_request_template.md

[ggkochanski]

• done

[ggkochanski]

It appears that property expression

@Parameter(property = "expression")
private String expression;

is already resolved when it goes to EvaluateMojo.execute()

e.g. executing
-Dexpression='project_version=${project.version}'

causes that the value of injected parameter expression is already resolved:
project_version=3.5.2-SNAPSHOT

and overlapping it with ${} does not do any good 😢

It overlaps already resolved expression with '${}' and tries to handle expression ${project_version=3.5.2-SNAPSHOT}, which fails with message

null object or invalid expression

However, the workaround exists 😛
passing property through additional parameter does the job, e.g.
-Dexpression='project_version=$${ver_property}' -Dver_property='{project.version}'
outputs desired value:
project_version=3.5.2-SNAPSHOT

[slachiewicz]

should we revert this change?

[ggkochanski]

should we revert this change?

Please keep it; it ultimately serves its purpose: you can prepare properties like format (or any other) within a single goal. This eliminates the need to use standard input, which is not always possible, e.g., during release