consider GOROOT/api as fallback to determine Go version

[ardnew]

When using a development branch of Go (see #435), the file $GOROOT/VERSION doesn't exist, and src/runtime/internal/sys/zversion.go contains a version string like the following:

const TheVersion = `devel +529988d62c Sun Mar 1 22:26:33 2020 +0000`

Instead of the expected format:

const TheVersion = `go1.13.7`

Thus tinygo will fail to run without any existing means of determining version, for example:

$ tinygo flash -target pyportal .
error: could not read version from GOROOT (/home/andrew/Development/go/dev/goroot): could not parse Go version: version does not start with 'go' prefix

This change will look at the content of directory $GOROOT/api to find the semantically greatest version only if the above two checks first fail, and the value of const TheVersion begins with "devel"

Note that this still won't allow current master branch to run, which will produce a version go1.14, resulting in:

$ tinygo flash -target pyportal .
error: requires go version 1.11, 1.12, or 1.13, got go1.14

But this is a separate unrelated issue (#901 I believe).

[deadprogram]

I think this has been addressed already. @ardnew can we close this PR?

[ardnew]

@deadprogram I'm not sure which commit would have addressed it, but I can test and verify it's resolved today and let you know.

I began always using a release version of Go specifically for my TinyGo runtime, which I think most people do anyway.

[ardnew]

Building master@HEAD from mainline Go, it will report a devel version number appended with a commit hash:

$ go version
go version devel go1.19-0c6d8bb109 2022/04/15 12:17:18 darwin/amd64

TinyGo doesn't immediately report any problems:

$ ./build/tinygo version
tinygo version 0.23.0-dev-2dc46a85 darwin/amd64 (using go version devel go1.19-0c6d8bb109 2022/04/15 12:17:18 and LLVM version 13.0.0)

But when actually trying to compile something, the following error is still generated:

$ ./build/tinygo build -target=arduino ./src/examples/blinky1/blinky1.go
error: could not read version from GOROOT (/usr/local/go/dev): could not parse Go version: version does not start with 'go' prefix

After rebasing this PR against latest dev branch, the logic in this change needed to be added to package goenv. It was also left in builder, but I'm not certain if this is used anymore?

Now building with the mainline Go development branch succeeds:

$ tinygo env
GOOS="darwin"
GOARCH="amd64"
GOROOT="/usr/local/go/dev"
GOPATH="/usr/local/go"
GOCACHE="/Users/andrew/Library/Caches/tinygo"
CGO_ENABLED="1"
TINYGOROOT="/Users/andrew/.tinygo"
$ tinygo version
tinygo version 0.23.0-dev-c3c09e9d darwin/amd64 (using go version devel go1.19-0c6d8bb109 2022/04/15 12:17:18 and LLVM version 13.0.0)
$ ./build/tinygo build -target=arduino -size=short ./src/examples/blinky1/blinky1.go
   code    data     bss |   flash     ram
   2354      90     673 |    2444     763