Once upon a time Go version strings for development builds did not
reference the major/minor version they belonged to, therefore the best
we could do was to always assume that a development build belonged to
the most recently released version of Go.
This commit changes pkg/goversion so that the new style of development
version string is fully parsed.
Add parsing for the new version format with toolchain (X.Y.Z-something)
and simplify internal representation of versions so that revision, beta
version and rc version are all represented on a single field with rc
and beta versions being negative numbers (this limits rc versions to a
maximum of 1000 which will not be a problem in practice).
This changes allow us to parse Go+BoringCrypto which formatted in <GoVersion>b<BoringCryptoVersion> so that we can surpress `Version of Go is too old for this version of Delve` error.
Fixes#2711
