When calling mage import, we weren't prefiltering the list of go
files, so we'd get files with build tags and test files and such.
What we want is just the normal go files for building the library.
Using go list returns us that, so use that.
This change adds `sanitizeSynopsis()` function to perform basic
sanitization on the synopsis. If synopsis begins with the function name, it
removes the function to avoid repeating words.
* Helpers galore
This commit adds helper functions that should be commonly needed
by many magefiles. It includes command execution handlers as well
as error handlers.
This commit also removes the only two non-stdlib dependencies in
the codebase. Since mage will likely be vendored into a repo,
it would be nice to avoid cluttering the vendor directory with
dependencies that are only for the build script.
