archive: remove pgzip, use gzip from go std lib (MR 25)

This replaces the parallel gzip with the boring gzip from Go's standard
lib. The main motivations for doing this are:

1) Reduce runtime memory requirements

2) shed some external dependencies

There's obviously a trade-off with compression speed/time (as seen
below), but I feel like it's a worthwhile trade-off.

Note that there's likely very little impact to boot performance wrt
extracting these archives, the compression levels are similar.

Measured on a Shift 6mq, which is a very fast phone...

** compress/gzip:

User time (seconds): 1.81
System time (seconds): 0.38
Percent of CPU this job got: 104%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.09
Maximum resident set size (kbytes): 62024

-rw-r--r-- 1 clayton clayton 6.1M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.5M Sep 20 17:20 initramfs-extra

** pgzip:

User time (seconds): 1.19
System time (seconds): 0.48
Percent of CPU this job got: 159%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.05
Maximum resident set size (kbytes): 109952

-rw-r--r-- 1 clayton clayton 6.8M Sep 20 17:20 initramfs
-rw-r--r-- 1 clayton clayton 2.8M Sep 20 17:20 initramfs-extra

inspired by: https://gitlab.com/postmarketOS/pmaports/-/issues/1704

go.mod

@@ -4,7 +4,5 @@ go 1.16
 
 require (
 	github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e
-	github.com/klauspost/compress v1.13.3 // indirect
-	github.com/klauspost/pgzip v1.2.5
 	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c
 )

go.sum

@@ -1,9 +1,4 @@
 github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e h1:hHg27A0RSSp2Om9lubZpiMgVbvn39bsUmW9U5h0twqc=
 github.com/cavaliercoder/go-cpio v0.0.0-20180626203310-925f9528c45e/go.mod h1:oDpT4efm8tSYHXV5tHSdRvBet/b/QzxZ+XyyPehvm3A=
-github.com/golang/snappy v0.0.3/go.mod h1:/XxbfmMg8lxefKM7IXC3fBNl/7bRcc72aCRzEWrmP2Q=
-github.com/klauspost/compress v1.13.3 h1:BtAvtV1+h0YwSVwWoYXMREPpYu9VzTJ9QDI1TEg/iQQ=
-github.com/klauspost/compress v1.13.3/go.mod h1:8dP1Hq4DHOhN9w426knH3Rhby4rFm6D8eO+e+Dq5Gzg=
-github.com/klauspost/pgzip v1.2.5 h1:qnWYvvKqedOF2ulHpMG72XQol4ILEJ8k2wwRl/Km8oE=
-github.com/klauspost/pgzip v1.2.5/go.mod h1:Ch1tH69qFZu15pkjo5kYi6mth2Zzwzt50oCQKQE9RUs=
 golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c h1:F1jZWGFhYfh0Ci55sIpILtKKK8p3i2/krTr0H1rg74I=
 golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

internal/archive/archive.go

@@ -5,7 +5,7 @@ package archive
 
 import (
 	"bytes"
-	"compress/flate"
+	"compress/gzip"
 	"fmt"
 	"io"
 	"log"
@@ -17,7 +17,6 @@ import (
 	"syscall"
 
 	"github.com/cavaliercoder/go-cpio"
-	"github.com/klauspost/pgzip"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/filelist"
 	"gitlab.com/postmarketOS/postmarketos-mkinitfs/internal/osutil"
 )
@@ -222,10 +221,7 @@ func (archive *Archive) writeCompressed(path string, mode os.FileMode) error {
 		return err
 	}
 
-	gz, err := pgzip.NewWriterLevel(fd, flate.BestSpeed)
-	if err != nil {
-		return err
-	}
+	gz := gzip.NewWriter(fd)
 
 	if _, err = io.Copy(gz, archive.buf); err != nil {
 		return err