This fixes a problem when --parallel-extensions-install is used when extensions are involved that can't actually be installed in parallel, like with Perl-5.34.1-GCCcore-11.3.0.eb:

$ eb Perl-5.34.1-GCCcore-11.3.0.eb --parallel-extensions-install
== installing extension File::Listing 6.15 (2/383)...
== ... (took 1 min 8 secs)
== FAILED: Installation ended unsuccessfully: Module command '/usr/share/lmod/lmod/libexec/lmod python load Perl/5.34.1-GCCcore-11.3.0' failed with exit code 1; stderr: Lmod has detected the following error: These module(s) or extension(s) exist but cannot be loaded as requested: "Perl/5.34.1-GCCcore-11.3.0"
   Try: "module spider Perl/5.34.1-GCCcore-11.3.0" to see how to load the module(s).

If you don't understand the warning or error, contact the helpdesk at [email protected]

EasyBuild will naively always try to install extensions in parallel when the --parallel-extensions-install configuration setting is enabled, and only give up on that when a NotImplementedError is raised; see EasyBlock.install_all_extensions.
A NotImplementedError will be raised by the install_extension_async method for extensions other than R packages.

We should probably look into changing that, and only start installing extensions in parallel (via EasyBlock.install_extensions_parallel) if all extensions that should be installed actually support it. If not, the extensions should just be installed sequentially (via EasyBlock.install_extensions_sequential).
Doing that correctly requires additional work though, probably an extra method in Extension that declares whether installing in parallel is supported, or not.

The acute problem here is that this leaves the environment in an inconsistent state, because the fake module that was loaded when the asynchronous installation of the 1st extension was being launched will not be cleaned up.

This makes Lmod quite unhappy as soon as an attempt is made to prepare the environment for the installation of the 2nd extension, see error above.

This fix in this PR makes sure that the fake module that was loaded is correctly cleaned up, and then there's no problem when the extensions are being installed.

edit: opened two issues to follow-up on things that were uncovered while working out this fix:

• fake modules should have a unique name #4896
• installing extensions in parallel should only be done when all extensions can be installed in parallel #4897