fix: ensure mounted installable packages are installed as expected

[DawoudSheraz]

Fixes #997

When xblocks and other installed packages are mounted, they are installed during image build. The dirs are copied over and then pip install takes place (resulting in egg-file generation). However, the dirs are not updated in host directories. When running tutor, the host files are re-mounted at run time. Due to missing egg-info, the installed packages no longer appear in pip list. This PR adds a small step during initialization that runs pip install on the mounted dirs if egg-info is missing.

Testing

• Checkout the branch & install.
• Clone scorm or any other xblock (as mentioned in tutor mounts add /path-to-openedx-scorm-xblock doesn't automatically install scorm-xblock #997)
• mount the xblock
• Re-build open edx image. This might not be needed if you already have recent image build. In that case, running tutor dev do init -l lms would be enough
• Bash into lms and run pip list | grep mnt. The mounted xblock(s) will show up.

[Danyal-Faheem]

LGTM! 👍
I've tested it out on Tutor version 17.0.2 and it's working perfectly.

[regisb]

Sweet!