Explicitly build Legion if legion_dir or legion_src_dir is not provided

[trxcllnt]

This PR fixes an issue where building and installing legate.core succeeds, but re-building and installing legate.core again (without uninstalling it first) seemingly succeeds but removes the Legion libs.

Here's the output from two runs:

• Build and install into a clean conda env
• Build and install into the same conda env again (w/o uninstalling legate.core)

1. The first run builds and installs Legion as part of building the legate.core wheel.
2. The second run doesn't build Legion (because the installed version from before was found by CMake)

As part of installing the wheel produced by step 2, pip first removes the files added by the previous installation (step 1), but it doesn't re-install Legion (because Legion was found in an install location and not built as a side-effect of building legate.core.)

This PR forces install.py to tell CMake to always build Legion as part of building legate.core unless the user explicitly passes --with-legion or --legion-src-dir.

[bryevdv]

This seems good to me

[bryevdv]

@trxcllnt I pushed the button to merge in the mypy fixes, should go green now :D

[bryevdv]

merge ok'd by @trxcllnt