change force rebuilding flags to -a

fix gopherjs serve command mod lookup

build: fix mod check skip goroot, rename buildImportPathWithSrcData t…

…o buildImportPathWithParent

build: use build context ReleaseTags slice that goes to go1.12 only

We want GopherJS 1.12-3 to have predictable behavior when building
packages regardless of the version of Go it was built with; it should
not be matching files with "// +build go1.13" constraints some times
but not other times.

Previously, ReleaseTags of the default build context were inherited
from the Go version used to build GopherJS, which may be newer than
Go 1.12.

This change makes it have ReleaseTags that go up to go1.12 always.
That corresponds to the major version of Go and its standard library
that is supported by GopherJS 1.12-3.

Fixes gopherjs#969.

GitHub-Pull-Request: gopherjs#970