Use pandacan package for panda

[premkiran2]

Validation PR for removing panda as an openpilot submodule and consuming it as a normal Python dependency.

This PR:

• removes the panda submodule entry from .gitmodules
• removes the panda gitlink
• replaces direct panda transport deps with pandacan from premkiran2/panda@proper-pip-package
• updates SConstruct to locate the installed panda package and run its packaged SConscript with a local variant_dir

Depends on panda package PR: commaai/panda#2403

Local validation:

• python3 -m py_compile SConstruct
• parsed pyproject.toml dependencies
• git diff --cached --check before commit

Full CI on this PR is the intended validation that openpilot can build without the panda submodule.