support for replace directives and dependency overrides

[myitcv]

It is currently impossible to install a fork of a project.

For example, github.com/myitcv/gopherjs is a fork of github.com/gopherjs/gopherjs. The module at the path github.com/myitcv/gopherjs is github.com/gopherjs/gopherjs, that is to say, the former can only be used as a replace target for the latter.

Hence gobin currently fails:

failed to run go get -d github.com/myitcv/gopherjs: exit status 1
go: finding github.com/myitcv/gopherjs latest
go: github.com/myitcv/gopherjs@v0.0.0-20181127201855-87822abcb5da: parsing go.mod: unexpected module path "github.com/gopherjs/gopherjs"
go: error loading module requirements

It should be possible to install the module that exists at path github.com/myitcv/gopherjs, perhaps via something like

$ gobin -replace github.com/gopherjs/gopherjs=github.com/myitcv/gopherjs github.com/gopherjs/gopherjs

Similarly, it would be nice to be able to support arbitrary dependency overrides. Say you want to install foo v1.0.0, but you know that there is a bug in bar v1.0.0. This might be made possible via something like:

gobin -require bar@v1.0.1 foo@v1.0.0

Things to consider above:

• if we support these require and replace augmentations then our caching strategy needs to change to be hash based, where the version, original go.mod (if there is one), additional replace and require directives all get hashed to give the build key
• ...