use lazyregexp for various regular expressions

[thaJeztah]

• alternative to use sync.OnceValue for various regular expressions #15

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

[codecov]

Codecov Report

❌ Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.02%. Comparing base (6ccba5a) to head (34ca3d7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/lazyregexp/lazyregexp.go	65.00%	7 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #25 +/- ##
==========================================
- Coverage 84.21% 83.02% -1.19% 
==========================================
Files 5 6 +1 Lines 304 324 +20 ==========================================
+ Hits 256 269 +13 - Misses 38 45 +7 
Partials 10 10 

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