github.com/rogpeppe/gohack

main: fix definition and implementation of relative GOHACK (#49)

rogpeppe has a far sharper brain than me and pointed out a major issue
with the previous implementation: that we hadn't actually implemented
a relative value of GOHACK relative to the main module.

Allow relative paths in GOHACK env var (#45)

Prior to this change, we were using `filepath.Join()` to derive a
replacement for the module dir. But this is not enough to
support relative directories, as `Join()` cleans the resulting path,
removing any `"./"` prefix (which is needed by the replace drective when
using a local directory).

This change adds the `"./"` prefix if the user specified it in the GOHACK
env var.

Note that relative parent directory `".."` is not cleaned by
`filepath.Join()`, so we do not need to cater to this case, and an added
test proves that this works.

fixes #44