improve handling of start_dir and add tests for cases where either ext_dir or initial start_dir or both are unset or None

[Flamefire]

Python packages may be installed from WHL files which do not get extracted in which case self.start_dir, self.ext_dir are both None. This leads to an unhelpful warning:

WARNING: Provided start dir (None) for extension tensorboard-plugin-wit does not exist:

Check that case and only log a debug message. Also add test for this.

Note that #4196 changed semantics: Prior to this start_dir or ext_dir where used if set and existing resolved relative to the current directory. After that start_dir is resolved relative to ext_dir which is usually the same but does not need to.

[lexming]

LGTM

I suggest to update the description of _set_start_dir to reflect the new logic

[lexming]

As discussed with @Flamefire, these are the target outcomes for _set_start_dir:

start_dir	ext_dir=None	ext_dir=relative path	ext_dir=absolute path
None	None	error (1)	absolute path
empty	set to build dir	error (1)	set to ext_dir
relative	absolute path from build dir	error (1)	absolute path from ext_dir
absolute	keep as is	error (1)	keep as is

1: might be done by some custom easyblock, cannot be handled as we do not know the base dir
2: always remove trailing slashes from absolute paths (except for start_dir='/'). This is needed for easyblocks that already rely on os.path.dirname() and to safely use a future start_dir template.

The result will be that start_dir is always an absolute path except where both ext_dir and start_dir are none (e.g. Python wheels)

[lexming]

LGTM

[lexming]

Test report with this PR and the develop branch of easyblocks and easyconfigs:
https://gist.github.com/lexming/fe9d940570f97d32f90011225f831238

Overview of tested easyconfigs (in order)

• SUCCESS immunedeconv-2.0.2-foss-2020a-R-4.0.0.eb : RPackage extensions with/without start_dir easyconfig parameter
• SUCCESS IJulia-1.24.0-Julia-1.8.5.eb : JuliaPackage extensions with/without start_dir easyconfig parameter
• SUCCESS isoCirc-1.0.4-foss-2020b.eb : PythonPackage extensions with/without start_dir easyconfig parameter
• SUCCESS IPython-8.5.0-GCCcore-11.3.0.eb : extensions without sources to unpack (PythonPackage with wheels)

[lexming]

Going in, thanks @Flamefire !