added support for basic file filtering when adding an explicit file

[fracpete]

• similar to the file filtering in the debian-maven-plugin plugin (BeanUtils, FilterUtils classes stem from that project)
• filtering gets activated for a file with the <filterFile>true</filterFile> tag
• the filtering replaces ${varname} variables with the actual values
• creates a tmp file with the expanded content, which gets deleted when the JVM exits
• offers access to project variables, like project.name (see src/site/markdown/entry.md)
• the following, additional variables are available:

  description
  distribution
  group
  packageName
  packager
  sourcePackage
  vendor
  version
  

• this allows having centralized script templates used by multi-module projects, with directories/etc being represented by variables and then expanded at build time, rather than duplicating the scripts

[ctron]

Sorry that this one has slipped my attention!

I don't really understand the reason for this PR though. Couldn't you simply use the maven filtering and then scoop the files with the RPM plugin?

[fracpete]

No worries, we're all busy. ;-)
The default resource filtering with Maven works fine with scripts that are part of the same module. However, when using the dependency plugin to unpack a dependency containing scripts (I reuse the same scripts with placeholders for a bunch of projects), then filtering seems to be impossible (at least, I haven't found a way). Hence the direct support for file-filtering as part of the entries. If you're still keen on including this functionality, then I'll close this PR and create a new one, since I've moved out the file-filtering into its own library. I use the same functionality in my forked plugin for generating debian packages.

[ctron]

So why don't you just use https://maven.apache.org/plugins/maven-resources-plugin/copy-resources-mojo.html instead?

I am not really fond of adding another dependency. Having a re-implementation of the filtering is also not really what I would like to see. I am also not sure about the license of that library. The source code says GPL, the project license APLv2.

[fracpete]

Most of my projects are GPLv3, hence I set up my IDE to automatically add the GPL preamble. I've fixed this. Thanks for pointing this out!
I will try to create minimal maven projects to see whether a combination of dependency-plugin/unpack-dependencies and maven-resources/copy-resources can be used to achieve the same result and will update on this.