Migrate to internal imports.

As of the Swift 6 compiler, `@_implementationOnly import` is deprecated in
favor of `internal import` and as a result the use of `@_implementationOnly
import` in this project is generating a lot of diagnostic noise when building
the Swift standard library.

For Swift libraries with library evolution, `@_implementationOnly import` and
`internal import` are roughly functionally equivalent, aside from improved
diagnostics for `internal import`. For non-resilient libraries, the main
difference is that `internal import` does not actually hide a module dependency
from downstream clients because the layout of a type in a non-resilient library
may depend on types coming from an `internal import` (with
`@_implementationOnly import` the same situation would result in a silent
mis-compile, which is the reason that `@_implementationOnly import` is
deprecated). The `_RegexParser` module dependency does not need to be hidden
from clients since it is installed in standard locations in the SDK/toolchain.
Therefore this migration should be safe, regardless of library resilience mode.

Author: tshortli