allow package projects to include themselves in the lockfile

[kevinushey]

Closes #2285.

Package projects can now request that the package itself be included in the lockfile, by setting the following in the package DESCRIPTION file:

Config/renv/snapshot/include-self: TRUE

When set:

• The package is no longer excluded from the lockfile during snapshot (same effect as the existing, previously-undocumented renv.snapshot.ignore.self option, which still takes precedence when set);
• The package is also treated as a dependency of the project itself, so inclusion doesn't depend on directory structure (e.g. an app.R referencing the package). This covers both implicit and explicit snapshot types.

An explicit Config/renv/snapshot/include-self: FALSE excludes the package even in golem projects, which are otherwise auto-included.

Also documents the renv.snapshot.ignore.self option in ?snapshot, along with the caveat that the package should be installed from a restorable remote source (e.g. renv::install("<user>/<repo>")) for deployment to work.