Module paths, like import paths, are case-sensitive, for better or worse.
But not all file systems distinguish file paths with different cases.
If we are going to use module paths to construct file system paths,
we must apply an encoding that distinguishes case without relying
upon the file system to do it.
This CL defines that encoding, the "safe module path encoding".
Module paths today are ASCII-only with limited punctuation,
so the safe module path encoding is to convert the whole path
to lower case and insert an ! before every formerly upper-case letter:
github.com/Sirupsen/logrus is stored as github.com/!sirupsen/logrus.
Although this CL defines the encoding, it does not change the rest
of the go command to use the encoding. That will be done in
follow-up CLs.