IssueTracker

We have a Go app, and we'd like to use the following app.yaml:
```
 runtime: go112
 service: service-name
 main: github.com/Khan/webapp/services/service-name/cmd/serve
```
(This is within a Go module github.com/Khan/webapp.)

Sadly, gcloud seems to prohibit it:
```
ERROR: (gcloud.app.deploy) An error occurred while parsing file: [/home/benkraft/khan/webapp/genfiles/go-staging/service-name/service-name.yaml]
Unable to assign value 'github.com/Khan/webapp/services/service-name/cmd/serve' to attribute 'main':
Value 'github.com/Khan/webapp/services/service-name/cmd/serve' for main does not match expression '^(?:[\w.\\\/:]+)$'
  in "/home/benkraft/khan/webapp/genfiles/go-staging/service-name/service-name.yaml", line 3, column 7
```

It's unclear to me why gcloud exposes such a restriction: this is a perfectly valid Go package name.  In fact, if I find where the check is happening -- MAIN_RE_STRING on lib/googlecloudsdk/third_party/appengine/api/appinfo.py:180 -- and edit it to allow dashes, the deploy works just fine -- this limitation seems to entirely be an unnecessarily strict client.

It would be great if gcloud allows these names!  We'd like to have our services in directories which match their deployed names which use hyphens.