Add a build flag for dlv debugging

[LiusCraft]

Proposal

Consider adding a gop build -dlv switch. When the -dlv switch exists, gop_autogen.go and <dlv_gop> are generated. For this dlv_gop, I expect <absolute path>/xxx.gop, if the -trimpath flag is carried during build, it is expected to be: <module path>/xxx.gop

Example:
absolute: /Users/qiniu/projects/testgop/xxx.gop
module: testgop/xxx.gop

Background

1. When setting breakpoints for debugging gop source files in the dlv debugger using the vscode client, specifying the source file location as an absolute path, the dap server cannot locate that location (currently it is a relative path).
2. If the current project imports another gop module, and they both have a utils/csgo.gop, then there would only be one in the sources, and the debugger would not be able to distinguish between them.

Workarounds

no