filer: add io/fs.FS Filer

[abursavich]

This adds a Filer implementation for the io/fs stdpkg introduced in go 1.16. It uses this implementation to replace the one used by FromDirectory. It also updates the version of go used in workflows to at least go 1.16.

[bzz]

Thank you @abursavich for the patch, it looks great to me. Will merge as soon as CI is 🟢

[bzz]

Windows tests seem to be failing for a while now 😞
Apparently that is at least partially due to us not handling platform-dependent newline char 🤦

• go-license-detector/licensedb/filer/filer_test.go

  Line 89 in 4f2ca6a

  assert.Equal(t, "world\n", string(content))

• go-license-detector/licensedb/filer/filer_test.go

  Line 32 in 4f2ca6a

  assert.Equal(t, "world\n", string(content))

• go-license-detector/licensedb/filer/filer_test.go

  Line 24 in 4f2ca6a

  assert.Equal(t, "hello\n", string(content))

That does not seem to be related to the change though, so 👍