Add some validation on version-var before passing it to Go

[ashrayjain]

What happened?

Sometimes version-var can have typos/incorrect package names after renames/moves/etc.

What did you want to happen?

Instead of silently dropping the version substitution, it would be much nicer UX if we errored/warned and alerted the user.

[nmiyake]

Work involved would be the following:

• Parse content of the version-var configuration to determine the package path and variable
  • Fail/error if format is not correct
• Load the target package using the Go packages API
  • Note that we need to consider the directory that is the build target as well, since "main" is a valid package identifier and, in that case, the directory location matters
• Look up the named variable to verify that it exists in the package
  • Possibly verify the type as well

Error output should clearly indicate what failed:

"unable to set version variable: package a.b.c does not exist"
"unable to set version variable: variable "version" not in package a.b.c"
"unable to set version variable: variable "version" is not of type string" (may need to verify if this is truly an error case)