enhance Tarball easyblock to support using it for installing extensions

[ItIsI-Orient]

(created using eb --new-pr)

[akesandgren]

Would be nice with an explanation of why your need this. I.e. what problem does it solve?

[ItIsI-Orient]

@akesandgren I am working on SCGid easyconfig and this software needs some libraries, that can be installed with tarball easyblock, but for some of the libraries, creating special easyconfig isn't the best idea (namely ClaMS-CLI, which only exists as a fork containing only the init commit). Therefore I would like to add them through exts_list

[ocaisa]

It sounds like a bundle with components would be useful for you. There's an example of what that looks like in https://github.com/easybuilders/easybuild-easyconfigs/pull/15018/files

[boegel]

Even though alternative approaches are possible here, I think this enhancement makes sense.

There have been other use cases where people have run into trouble when trying to use Tarball to install an extension, so even if we don't end up using this for SCGid, it still makes sense imho.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS Trimmomatic-0.39-Java-11.eb
• SUCCESS snippy-4.6.0-GCC-10.2.0.eb
• SUCCESS Spark-3.1.1-foss-2020b.eb
• SUCCESS SpaceRanger-1.3.0-GCC-10.3.0.eb
• SUCCESS Net-core-3.0.0.eb
• SUCCESS Julia-1.7.2-linux-x86_64.eb

Build succeeded for 6 out of 6 (6 easyconfigs in total)
node2607.swalot.os - Linux CentOS Linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 3.6.8
See https://gist.github.com/6f38b66baaa444fb3667f4fd2f044375 for a full test report.