This patch primarily adds more type hints for `rdflib.graph`, but also
adds type hints to some related modules in order to work with the new
type hints for `rdflib.graph`.

I'm mainly doing this as a baseline for adding type hints to
`rdflib.store`.

I have created type aliases to make it easier to type everything
consistently and to make type hints easier easier to change in the
future. The type aliases are private however (i.e. `_`-prefixed) and
should be kept as such for now.

This patch only contains typing changes and does not change runtime
behavior.

Broken off from https://github.com/RDFLib/rdflib/pull/1850

for more information, see https://pre-commit.ci

for more information, see https://pre-commit.ci

Plugin loading in `rdflib.plugins.sparql` will now happen similar to
what is being done for `rdflib.plugins`.

This also eliminates a warning that occurs `rdflib.plugins.sparql` and
makes it possible to enable `warn_unused_ignores` without any exceptions
or workarounds.

Also:
- Removed unused `type: ignore` statements.