Move "GetPathRelativeTo" from terragrunt and patcher here as a utility function

[ina-stoyanova]

So far, we have had to copy and paste the GetPathRelativeTo function from terragrunt repo to the patcher/util repo package. We should instead have this command available here so we avoid duplication.

[infraredgirl]

This function already exists in go-commons: https://github.com/gruntwork-io/go-commons/blob/master/files/paths.go#L70.

However, it seems that a bug fix has been implemented in the function in Terragrunt, and hasn't been ported here.

Therefore the right course of action is:

• Port the above bug fix from Terragrunt to go-commons
• Remove the definition of GetPathRelativeTo from Terragrunt (and any other repos that currently define their own versions of it) and use the function from go-commons instead

[infraredgirl]

Another bug in GetPathRelativeTo() was discovered and I just posted a fix in #78. After this is merged, I'll release another version of go-commons and ensure that all the repos that start to make use of GetPathRelativeTo() use this latest version.

[infraredgirl]

Another bug in GetPathRelativeTo() was discovered and I just posted a fix in #78. After this is merged, I'll release another version of go-commons and ensure that all the repos that start to make use of GetPathRelativeTo() use this latest version.

This has been done.