Proposal: improve `riot run` building of base package

[Kyle-Verhoog]

Currently riot run will always build the dev package of the project. For Python packages that compile + install C extensions this can be really expensive and unnecessary.

Proposals

1. riot run installs the dev package if not initially installed but doesn't otherwise install.

2. riot run doesn't install the dev package by default. (Maybe provide a --build or --generate flag to do so)