all: use GOMODCACHE where appropriate

[hyangah]

go1.15 is introducing GOMODCACHE.
Audit for assumptions that the module cache is in GOPATH[0]/pkg/mod and fix them to use GOMODCACHE when it's available.

e.g.
https://github.com/golang/vscode-go/blob/master/src/util.ts#L488
https://github.com/golang/vscode-go/blob/master/src/debugAdapter/goDebug.ts#L1081

cc/ @quoctruong @polinasok @stamblerre

[hyangah]

Also see golang/go#39761

[gopherbot]

Change https://golang.org/cl/240695 mentions this issue: src/util: use GOMODCACHE if available

[hyangah]

I am afraid the fix for goDebug.ts is a bit more involved. The running binary is compiled in a different setting, and we can't make any assumption about the module cache. The only invariance is the package import path (and version string).
@quoctruong

[hyangah]

It doesn't seem like to happen now. Moving out of v0.17.0 milestone.

[hyangah]

Closing because the only remaining issue is to fix goDebug.ts and we are going to officially deprecate the legacy debug adapter.