defer hatch toml parsing out of configure step (#469)

fixes #462

Hatch::configure() previously parsed pyproject.toml and hatch.toml for
every workspace folder on every call. That imposed Hatch's per-workspace
TOML cost on every user — including the majority who never use Hatch.

This PR switches Hatch to the same lazy-parse pattern pet-poetry and
pet-uv already use

- Non-Hatch users pay nothing
- Hatch users absorb the cost where it belongs — refresh. That's an
operation users are already waiting on; a one-time parse-per-workspace
is invisible against the rest of discovery.
- Convention alignment. Hatch was the only locator doing TOML I/O in
configure()

Author: eleanorjboyd