Downgrade yaegi to v0.9.19

[ldez]

What does this PR do?

• downgrade yaegi to v0.9.19
• fix mapping

Motivation

Fix regression with v0.9.20

More

• Added/updated tests
• Added/updated documentation

Additional Notes

DEBU[2021-07-19T18:10:50+02:00] loading of plugin: example: github.com/traefik/pluginproviderdemo@v0.2.0 
12:6: panic
panic: reflect.Set: value of type *struct { Xname string; XpollInterval time.Duration; Xcancel func() } is not assignable to type func() error [recovered]
        panic: reflect.Set: value of type *struct { Xname string; XpollInterval time.Duration; Xcancel func() } is not assignable to type func() errorgoroutine 1 [running]:
github.com/traefik/yaegi/interp.runCfg.func1(0xc000ed7e40, 0xc000120d00, 0xc000b72d68)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/yaegi@v0.9.20/interp/run.go:185 +0x253
panic(0x304ec80, 0xc000efb030)
        /home/ldez/.gvm/gos/go1.16.6/src/runtime/panic.go:965 +0x1b9
reflect.Value.assignTo(0xc000ae0a80, 0xc0006b0d68, 0x196, 0x3829949, 0xb, 0x3023900, 0x0, 0x32c8440, 0x0, 0xc0002759c8)
        /home/ldez/.gvm/gos/go1.16.6/src/reflect/value.go:2451 +0x3f7
reflect.Value.Set(0x3023900, 0xc0002759c8, 0x193, 0xc000ae0a80, 0xc0006b0d68, 0x196)
        /home/ldez/.gvm/gos/go1.16.6/src/reflect/value.go:1564 +0xbd
github.com/traefik/yaegi/interp.genInterfaceWrapper.func1(0xc000ed7e40, 0xc000efb020, 0x16, 0x32fca20)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/yaegi@v0.9.20/interp/run.go:1004 +0x1e5
github.com/traefik/yaegi/interp.assign.func3(0xc000ed7e40, 0xc000da7b00)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/yaegi@v0.9.20/interp/run.go:631 +0x13e
github.com/traefik/yaegi/interp.runCfg(0xc000120d00, 0xc000ed7e40)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/yaegi@v0.9.20/interp/run.go:191 +0x87
github.com/traefik/yaegi/interp.genFunctionWrapper.func2.1(0xc000ae3090, 0x3, 0x3, 0xc000ae3090, 0x3469e0, 0x30ce9e0)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/yaegi@v0.9.20/interp/run.go:952 +0x3e8
reflect.Value.call(0xc000272780, 0xc000da7dd0, 0x13, 0x381466b, 0x4, 0xc000b73498, 0x3, 0x3, 0xc000b73378, 0x4104bb, ...)
        /home/ldez/.gvm/gos/go1.16.6/src/reflect/value.go:476 +0x8e7
reflect.Value.Call(0xc000272780, 0xc000da7dd0, 0x13, 0xc000b73498, 0x3, 0x3, 0xc000272780, 0xc000da7dd0, 0x13)
        /home/ldez/.gvm/gos/go1.16.6/src/reflect/value.go:337 +0xb9
github.com/traefik/traefik/v2/pkg/plugins.newProvider(0xc0005002d0, 0x26, 0xc000bdd590, 0x25, 0x0, 0x0, 0xc00000c1e0, 0xc000502390, 0xc00027cdd0, 0xe, ...)
        /home/ldez/sources/go/src/github.com/traefik/traefik/pkg/plugins/providers.go:126 +0x9f0
github.com/traefik/traefik/v2/pkg/plugins.Builder.BuildProvider(0xc000b26ba0, 0xc000b26bd0, 0xc0003d14b0, 0x7, 0xc000502390, 0x3db5f98, 0xc000537b30, 0x0, 0x0)
        /home/ldez/sources/go/src/github.com/traefik/traefik/pkg/plugins/providers.go:63 +0x1ad
main.setupServer(0xc000124300, 0xc000537920, 0x1e, 0xc0007131c0)
        /home/ldez/sources/go/src/github.com/traefik/traefik/cmd/traefik/traefik.go:238 +0x657
main.runCmd(0xc000124300, 0x0, 0x0)
        /home/ldez/sources/go/src/github.com/traefik/traefik/cmd/traefik/traefik.go:122 +0x3a5
main.main.func1(0xc00004e210, 0x0, 0x0, 0x0, 0xc000124380)
        /home/ldez/sources/go/src/github.com/traefik/traefik/cmd/traefik/traefik.go:63 +0x2c
github.com/traefik/paerser/cli.run(0xc000124380, 0xc00004e210, 0x0, 0x0, 0x80, 0xc000403750)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/paerser@v0.1.4/cli/commands.go:133 +0x171
github.com/traefik/paerser/cli.execute(0xc000124380, 0xc00004e210, 0x1, 0x1, 0xc000000101, 0x200000003, 0xc000000180)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/paerser@v0.1.4/cli/commands.go:57 +0x8ed
github.com/traefik/paerser/cli.Execute(...)
        /home/ldez/sources/go/pkg/mod/github.com/traefik/paerser@v0.1.4/cli/commands.go:51
main.main()
        /home/ldez/sources/go/src/github.com/traefik/traefik/cmd/traefik/traefik.go:79 +0x41d
exit status 2

[jbdoumenjou]

LGTM