* Add support for rc and beta versions in go.mod file
The regex in `parseGoVersionFile()` didn't consider rc and beta
versions. The regex has been extended.
In `findMatch()` the match is checked using `semver.satisfies()`. But
`semver.satisfies()` doesn't no about Go's non-semver rc and beta
version formats (1.16c1 instead of 1.16.0-rc.1).
We cannot use `makeSemver()` on `versionSpec` and compare the result
using `semver.satisfies()` because `versionSpec` could be, well, a
spec. Instead we first check if there is a strict match between the
candidate version and the versionSpec.
Fixes#525.
* Add support for beta versions in go.mod toolchain directive
* Support prerelease versions in the toolchain cache
Arne Jørgensen