Implement manual registry

[certik]

The details are discussed at:

https://github.com/fortran-lang/fpm/issues/34

We should have a TOML file with a package name and a version / url combo for each version, something like:

[package1]
"0.1.5" = "https://somewhere/package1-0.1.5.tar.gz"
"0.1.6" = "https://somewhere/package1-0.1.6.tar.gz"

[stdlib]
"1.0.0" = {git="htttps://github.com/fortran-lang/stdlib", tag="v1.0.0"}
"latest" = {git="htttps://github.com/fortran-lang/stdlib", branch="master"}  # This would checkout the latest master

We can discuss if we should allow the "latest" --- perhaps we should simply only allow specific versions, that seems to be the case with other package managers.

[certik]

We now have the initial TOML file (https://github.com/fortran-lang/fpm-registry/blob/235aebc6968408e2e5468168d2873aaeef7cd07a/registry.toml), now we need to implement CI that will check that it is formatted correctly, and then to generate our rich metadata JSON file.

[certik]

Regarding the "latest" --- I think we should allow it. I think it will allow fpm to download the latest development version of the package (and then lock the particular latest git commit in fpm.lock). So fpm search can list all versions plus the latest and users can choose.