Improve performance by factoring out pkg_resources

[asottile]

Resolves #840

This saves ~100ms on startup from some naive tests:

before

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.363s
user	0m0.303s
sys	0m0.043s

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.359s
user	0m0.319s
sys	0m0.032s

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.384s
user	0m0.330s
sys	0m0.044s

after

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.253s
user	0m0.221s
sys	0m0.028s

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.254s
user	0m0.229s
sys	0m0.020s

$ time pre-commit --version
pre-commit 1.11.2

real	0m0.256s
user	0m0.219s
sys	0m0.024s