add deprecation notice to RPackage extensions with relative paths in start_dir

[lexming]

Follow-up to #2867 and easybuilders/easybuild-framework#4206

As discussed in the last conf call, we will re-instate the old behaviour with a deprecation notice in case somebody upgrades their easybuild-easyblocks without upgrading easybuild-framework.

[lexming]

Example deprecation warning on old framework (<=4.7.0) for extensions using a relative start_dir:

== 2023-02-08 15:50:39,936 build_log.py:147 WARNING Deprecated functionality, will no longer work in v5.0: Found relative path in start_dir, please upgrade to easybuild-framework>=4.7.1; see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS DUBStepR-1.2.0-foss-2021b-R-4.1.2.eb
• SUCCESS inferCNV-1.3.3-foss-2021a.eb
• SUCCESS rjags-4-13-foss-2022a-R-4.2.1.eb
• SUCCESS scPred-1.9.2-foss-2021b-R-4.1.2.eb
• SUCCESS SeuratDisk-0.0.0.9020-foss-2022a-R-4.2.1.eb
• SUCCESS R-4.2.1-foss-2022a.eb
• SUCCESS R-bundle-Bioconductor-3.15-foss-2022a-R-4.2.1.eb

Build succeeded for 7 out of 7 (7 easyconfigs in total)
node3104.skitty.os - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/2069fe37ae5667faf7bd7ae93c67f5df for a full test report.