Use "." in install() instead of DistributionFormat

[cjolowicz]

Use the same type signature for install that nox.Session.install uses.

The function no longer accepts DistributionFormat values in its arguments list. Use installroot instead.

Rationale

Besides the advantages of symmetry, this will allow us to handle extras for the local package. The enum-based interface does not allow expressing extras. By moving the translation logic from the patched Session.install function to install, we are able to pass arguments like ".[extra]". Interpreting extras as appropriate is left for a follow-up.