add generic JuliaPackage easyblock

[lexming]

Easyblock for Julia packages:

• Installs packages from local sources
• Sources can either be a tarball or a git repo (git_config)
• Avoids any extra downloads (offline mode)
• Updates JULIA_DEPOT_PATH to make Julia aware of the installation
• defines EXTS_FILTER_JULIA_PACKAGES to sanity check the installation
• Works for regular easyconfigs and also for extensions
• extra option bundle_deps enables Julia to download extra dependencies on-the-fly (opt-in needed to use this easyblock with Julia < 1.5)

Julia needs a git repo to install any package in a regular way because it generates unique hashes for the package from it. Hence, this easyblock detects what kind of sources we have and acts accordingly:

• git repo: install as regular package with Pkg.add
• non-git tarball: install in development mode with Pkg.develop

[lexming]

Requested new tests in the linked easyconfigs.

[boegel]

lgtm