let Bundle easyblock pick up custom easyblock for components based on name if no easyblock is specified explicitly

[boegel]

Without this change, you always get No easyblock specified for component if default_easyblock is not set and no explicit easyblock is specified for a component, even though there might be a custom easyblock already for that specified component.

We're already calling out to get_easyblock_class which will look for a custom easyblock if the easyblock value is None, but we're giving up too early currently...

[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/2d4b7f7105a2b0f8c4b9536671fb7084 for a full test report.

[branfosj]

lgtm

[branfosj]

Going in, thanks @boegel!