correctly determine easyblock to use for Bundle components

[boegel]

The changes in #2543 were clearly not tested enough, the cause trouble in case default_easyblock is specified, but no custom easyblock is specified for a component (as is the case for jaxlib in easybuilders/easybuild-easyconfigs#13760)

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS SentencePiece-0.1.96-GCC-10.2.0.eb
• SUCCESS X11-20210518-GCCcore-10.3.0.eb
• SUCCESS Xvfb-1.20.9-GCCcore-9.3.0.eb
• SUCCESS PyQt5-5.15.4-GCCcore-10.3.0.eb
• SUCCESS BioPP-2.4.1-GCC-9.3.0.eb
• SUCCESS MAFFT-7.453-gompi-2020a-with-extensions.eb
• SUCCESS NGS-2.10.9-GCCcore-10.2.0.eb

Build succeeded for 7 out of 7 (7 easyconfigs in total)
node3170.skitty.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, Python 3.6.8
See https://gist.github.com/4b0306489154e9881222e4b35a36e9ae for a full test report.

[branfosj]

This is working as expected for jax (easybuilders/easybuild-easyconfigs#13760 (comment)).

[branfosj]

lgtm

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS SentencePiece-0.1.96-GCC-10.2.0.eb
• SUCCESS X11-20210518-GCCcore-10.3.0.eb
• SUCCESS Xvfb-1.20.9-GCCcore-9.3.0.eb
• SUCCESS BioPP-2.4.1-GCC-9.3.0.eb
• SUCCESS MAFFT-7.453-gompi-2020a-with-extensions.eb
• SUCCESS NGS-2.10.9-GCCcore-10.2.0.eb

Build succeeded for 6 out of 6 (6 easyconfigs in total)
bear-pg0211u03a.bear.cluster - Linux RHEL 8.3, x86_64, Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz (cascadelake), Python 3.6.8
See https://gist.github.com/049f07be6b9aca250d6ac891446741e5 for a full test report.

[branfosj]

Going in, thanks @boegel!