tools: invalid value in go.alternateTools confuses tool search

[hyangah]

Originally reported by @dr2chase

• How to repro: provide an invalid path for go. For example, the user has a go binary in $HOME/work/go/bin/go but VSCode doesn't resolve $HOME automatically (~ is resolved though). So, the following setting won't work as expected.

  "go.alternateTools": {
     "go": "$HOME/work/go/bin/go"
  }

• What do I expect to happen:
  Clear error message pointing out the problem with this setting.

• What did I see instead:
  Confusing error message ""Failed to find the "go" binary in either GOROOT(/Users/drchase/work/go-1.15) or PATH(/Users/drchase/bin:<omitted>).Check PATH, or Install Go and reload the window. If PATH isn't what you expected, see https://github.com/golang/vscode-go/issues/971"

• What happened:

"go.alternateTools" supports both
case A) the absolute path to the alternate tool, and
case B) an alternative tool name (e.g. instead of go, maybe look for go-1.15)

When resolving the path for go, the extension called getBinPathWithPreferredGopathGorootWithExplanation with alternateTool = '$HOME/work/go/bin/go'. $HOME isn't expanded, so it doesn't look like an absolute path nor it's a valid executable path

vscode-go/src/utils/pathUtils.ts

Line 63 in b79bdf9

if (alternateTool && path.isAbsolute(alternateTool) && executableFileExists(alternateTool)) {

As a result, the extension thinks the user's intention was to provide an alternative tool name (case B).

Thus, in the following tool search path, the extension tries to look up '$HOME/work/go/bin/go' from all the paths (GOBIN, GOROOT/bin, PATH, ...), and miserably fails.

When a user supplies go.alternateTools and the lookup failed.

• If the path contains directory path separators, consider this was a misconfigured case A.
• Otherwise, show the binname used in the actual search.

cc @dr2chase @findleyr @suzmue

Re: $HOME. Not sure if we want to expand $HOME as we already do for ~. VSCode has variables for tasks/launch settings. I feel like we need to stop reinventing the wheel and look into a way to support more standard solution instead. (i.e. ${env:HOME}). https://code.visualstudio.com/docs/editor/variables-reference#_environment-variables