env: allow compiling code outside llgo directory

[cpunion]

Fix: #892 #651, need update release distribution and installation to use it.

Supports:

• env LLGO_ROOT: set to llgo root dir that can find go.mod contains module github.com/goplus/llgo\n
• Directory that contains the executable in bin/, and also has go.mod contains module github.com/goplus/llgo\n
• Will support go build -ldflags "-X github.com/goplus/llgo/compiler/internal/env.LLGO_ROOT=/path/to" after separate compiler and library

And it will support separation runtime/patches from llgo library.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.65%. Comparing base (7f10bc2) to head (290f99b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #933   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          22       22           
  Lines        6313     6313           
=======================================
  Hits         6165     6165           
  Misses        128      128           
  Partials       20       20           

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