This commit allows version specifiers to refer to git branches that contain
forward slashes. For example, the following is valid syntax now:
pkg@git.releases/1.0
It also adds a new method `Spec.format_path(fmt)` which is like `Spec.format`,
but also maps unsafe characters to `_` after interpolation. The difference is
as follows:
>>> Spec("pkg@git.releases/1.0").format("{name}/{version}")
'pkg/git.releases/1.0'
>>> Spec("pkg@git.releases/1.0").format_path("{name}/{version}")
'pkg/git.releases_1.0'
The `format_path` method is used in all projections. Notice that this method
also maps `=` to `_`
>>> Spec("pkg@git.main=1.0").format_path("{name}/{version}")
'pkg/git.main_1.0'
which should avoid syntax issues when `Spec.prefix` is literally copied into a
Makefile as sometimes happens in AutotoolsPackage or MakefilePackage
9cde25b39e