V3: The version of goa specified in go.mod is ignored and rewritten

[ikawaha]

According to The go blog: Using Go modules, go get changes the required version of a dependency (or adds a new dependency).

Since goa gen (or goa example) executes go get at runtime,
even if the goa version is specified in the go.mod file, it is ignored and rewritten.

Isn't this behavior causing confusion? There is also a problem that the latest
goa specified by the latest commit number can not be used.

Example:

before
go.mod

module github.com/ikawaha/calc

go 1.12

require (
...snip...
        goa.design/goa/v3 v3.0.3-0.20190704022140-85024ebc66dc  // latest version
)

after goa gen/example

$ goa gen calc/design

module github.com/ikawaha/calc

go 1.12

require (
...snip...
        goa.design/goa/v3 v3.0.2
)

[raphael]

Thank you for the fix!