In this commit, we turn the package into a new Go module (version 2),
and then port over the current set of types and functions to mainly
alias to the more optimized and maintained dcrec variant.
Taking a look at the benchmarks, most operations other than
normalization (which IIRC is a bit slower now due to constant time
fixes) enjoy some nice speeds up:
```
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark old ns/op new ns/op delta
BenchmarkAddJacobian-8 464 328 -29.20%
BenchmarkAddJacobianNotZOne-8 1138 372 -67.27%
BenchmarkScalarBaseMult-8 47336 31531 -33.39%
BenchmarkScalarBaseMultLarge-8 42465 32057 -24.51%
BenchmarkScalarMult-8 123355 117579 -4.68%
BenchmarkNAF-8 582 168 -71.12%
BenchmarkSigVerify-8 175414 120794 -31.14%
BenchmarkFieldNormalize-8 23.8 24.4 +2.39%
BenchmarkParseCompressedPubKey-8 24282 10907 -55.08%
```
Putting the test code in the same package makes it easier for forks
since they don't have to change the import paths as much and it also
gets rid of the need for internal_test.go to bridge.
Also, remove the exception from the lint checks about returning the
unexported type since it is no longer required.
