x/tools/gopls: "no package for ..." even after added to go.mod

[JAicewizard]

What version of Go are you using (go version)?

$ go version
go version go1.13.5 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/jaap/.cache/go-build"
GOENV="/home/jaap/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/jaap/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/jaap/go-projects/generator/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build223975312=/tmp/go-build -gno-record-gcc-switches"

What did you do?

puts package in import -> no package for ...
go mod tidy -> no package for ...
go mod vendor (without previously existing vendor folder) -> no package for ...
reloading file -> no error
removing the import statement and adding it back -> no error

What did you expect to see?

there should be no error in the first place since it is a valid package and after go mod tidy even has a version attached to it

[stamblerre]

To clarify, this package was not yet in the module cache?

[JAicewizard]

This module is completely new, and doing the same thing with the same module again wont trigger this behavior.
After clearing the module cache using go clean --modcache this behavior can be replicated again with the same module.
So yes this has probably something to do with module cache.