feat(gengo): support convert go+ files into go code

[CarlJi]

This PR primarily addresses the following two issues.

• gop go doesn't support convert go+ files into go, fox example:

➜  101-Hello-world git:(main) ✗ gop go -v hello-1.gop
2024/02/02 16:53:22 SetDebug: import=true, match=true, instr=true
2024/02/02 16:53:22 `gop go` doesn't support *gopprojs.FilesProj
panic: `gop go` doesn't support *gopprojs.FilesProj


goroutine 1 [running]:
log.Panicln({0xc000205e28?, 0xc0000ba060?, 0x1d045b8?})
	/opt/hostedtoolcache/go/1.21.6/x64/src/log/log.go:446 +0x5a
github.com/goplus/gop/cmd/internal/gengo.runCmd(0xc0000d8000?, {0xc0000ba060?, 0x0?, 0xc00006a688?})
	/home/runner/work/gop/gop/cmd/internal/gengo/go.go:92 +0x365
main.main()
	/home/runner/work/gop/gop/cmd/gop/main.go:113 +0x4b3

• The original single file mode flag -s seems aim to work in package mode, so i update the description accordingly:

➜  101-Hello-world git:(main) ✗ ll
total 24
drwxr-xr-x   5 jicarl  staff   160 Feb  2 16:58 .
drwxr-xr-x  63 jicarl  staff  2016 Feb  2 16:45 ..
-rw-r--r--   1 jicarl  staff   358 Dec 15  2021 hello-1.gop
-rw-r--r--   1 jicarl  staff   469 Dec 15  2021 hello-2.gop
-rw-r--r--   1 jicarl  staff   776 Dec 15  2021 hello-3.gop
➜  101-Hello-world git:(main) ✗ gop go -s .
Single file mode
GenGo hello-1.gop ...
GenGo hello-2.gop ...
GenGo hello-3.gop ...
➜  101-Hello-world git:(main) ✗

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1d25500) 91.90% compared to head (92ec1b4) 91.90%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1704   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files          39       39           
  Lines       10750    10750           
=======================================
  Hits         9880     9880           
  Misses        704      704           
  Partials      166      166           

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