go module proof go:generate protoc

[gdbelvin]

go:generate does not invoke bash by default, it merely does some basic variable
substitution and then calls execute directly.

Go modules deprecates $GOPATH, requiring a layer of indirection to look up the
location of proto files in 3rd party repositories.

By calling "sh", we can get proper variable substitution.

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The proto files themselves have been modified to remove any $GOPATH specifics, leaving the resolution of the package location to go list.

This PR adds option go_package to the protos to remove ambiguity, since their paths on disk are no longer required to be in $GOPATH, we cannot rely on their paths on disk to derive their go package names.

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An implication of making these import paths project relative is that anyone attempting to use grpc_cli will need to replicate each -I on the commandline to help the tool resolve dependencies.

Previously, passing $GOPATH/src may have been sufficient as long as the -I . import was not used in a particular proto file.

[codecov]

Codecov Report

Merging #535 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #535   +/-   ##
=======================================
  Coverage   71.92%   71.92%           
=======================================
  Files          88       88           
  Lines        9550     9550           
=======================================
  Hits         6869     6869           
  Misses       2190     2190           
  Partials      491      491

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[RJPercival]

As discussed in person, --go_out still using $GOPATH means that certificate-transparency-go will have to be checked out under $GOPATH (i.e. it can't be outside of the Go source tree like modules usually can). We should document this restriction, since we're not currently aware of a way to avoid it.