env: refactor all the things

The `IsolatedEnv` has been reworked to subsume env creation
logic.

`IsolatedEnvBuilder` renamed to `IsolatedEnvManager`.

The `IsolatedEnv` takes IsolatedEnvManager` as an argument
instead of passing the manager's attributes to the env piecemeal.

`ProjectBuilder.from_isolated_env` has been added to auto-fill
init args from an isolated env.

The `ProjectBuilder`'s properties were made read-only. Mutating
the builder is not necessary with the addition of `from_isolated_env`.

Factored out parsing `pyproject.toml`.

Fixed passing the absolute path to the srcdir validator in the project
builder.

Reworded `ProjectBuilder` docstrings for clarity.

Moved runner logic to the isolated env as prelude to fixing pypa#377.
As a consequence the runner is not replaceable when using isolation.

Removed pip version detection.  The default env updates pip in a subprocess.