debug: allow to configure `substitutePath` for debug test codelens and symlinks

[hyangah]

Users who work in symlinked workspace need to use substitutePath (introduced as a fix for #622) because VSCode may use the symlinked path
while the Go command and Delve would use the original path.

This solution is not available for debug sessions started with the debug test codelens or
the debug session that started with a default configuration without launch.json (see #855)

A couple of options:

• allow users to configure the default substitutePath settings (e.g. go.delveConfig ) in settings.json
• try to detect this symlink case and auto-populate substitutePath automatically.
• when this case is detected, the extension pops up a message to ask the user to configure the setting (or even update the settings value on behalf of the users).

[gopherbot]

Change https://golang.org/cl/318589 mentions this issue: src/goDebugConfiguration.ts: add substitutePath to go.delveConfig

[hyangah]

Since v0.26.0, substitutePath can be set using go.delveConfig in settings.json.
@suzmue Do you want to keep looking into this problem (e.g. auto-generate substitutePath mapping, or provide a way for users to inspect the file path info embedded in the debugged binary and configure the mappings on the fly, ...)? I think it's ok to close this issue, and open separate issues to investigate alternative options to improve UX. What do you think?

[suzmue]

Closing this since this particular issue is resolved. We can open separate issues for follow up features.