Using regex.MustCompile consumes a significant amount of memory when
importing the package, even if those regular expressions are not used.

This changes compiling the regular expressions to use a lazyregexp
package so that they're only compiled the first time they're used.

There are various regular expressions remaining that are still compiled
on import, but these are exported, so changing them to a sync.OnceValue
would be a breaking change; we can still decide to do so, but leaving
that for a follow-up.

To verify, compile a basic binary importing the package;

    package main

    import _ "github.com/distribution/reference"

    func main() {}

Before:

    for i in $(seq 1 5); do GODEBUG=inittrace=1 ./before 2>&1 | grep distribution/reference; done

    init github.com/distribution/reference @0.94 ms, 0.22 ms clock, 415712 bytes, 3599 allocs
    init github.com/distribution/reference @0.39 ms, 0.22 ms clock, 415712 bytes, 3599 allocs
    init github.com/distribution/reference @0.39 ms, 0.23 ms clock, 415712 bytes, 3599 allocs
    init github.com/distribution/reference @0.45 ms, 0.27 ms clock, 415712 bytes, 3599 allocs
    init github.com/distribution/reference @0.38 ms, 0.24 ms clock, 415712 bytes, 3599 allocs

After:

    for i in $(seq 1 5); do GODEBUG=inittrace=1 ./after 2>&1 | grep distribution/reference; done

    init github.com/distribution/reference/internal/lazyregexp @0.85 ms, 0 ms clock, 0 bytes, 0 allocs
    init github.com/distribution/reference @1.0 ms, 0.16 ms clock, 238680 bytes, 1383 allocs
    init github.com/distribution/reference/internal/lazyregexp @0.33 ms, 0 ms clock, 0 bytes, 0 allocs
    init github.com/distribution/reference @0.42 ms, 0.16 ms clock, 238680 bytes, 1383 allocs
    init github.com/distribution/reference/internal/lazyregexp @0.39 ms, 0 ms clock, 0 bytes, 0 allocs
    init github.com/distribution/reference @0.47 ms, 0.19 ms clock, 238680 bytes, 1383 allocs
    init github.com/distribution/reference/internal/lazyregexp @0.36 ms, 0 ms clock, 0 bytes, 0 allocs
    init github.com/distribution/reference @0.47 ms, 0.14 ms clock, 238680 bytes, 1383 allocs
    init github.com/distribution/reference/internal/lazyregexp @0.29 ms, 0 ms clock, 0 bytes, 0 allocs
    init github.com/distribution/reference @0.38 ms, 0.15 ms clock, 238680 bytes, 1383 allocs

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>